Antioxidant stabilized crosslinked ultra-high molecular weight polyethylene for medical device applications

ABSTRACT

An antioxidant combined with UHMWPE prior to subjecting the UHMWPE to crosslinking irradiation. In one exemplary embodiment, the antioxidant is tocopherol. After the antioxidant is combined with the UHMWPE, the resulting blend may be formed into slabs, bar stock, and/or incorporated into a substrate, such as a metal, for example. The resulting product may then be subjected to crosslinking irradiation. In one exemplary embodiment, the UHMWPE blend is preheated prior to subjecting the same to crosslinking irradiation. Once irradiated, the UHMWPE blended product may be machined, packaged, and sterilized in accordance with conventional techniques.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under Title 35 U.S.C. § 119(e) ofU.S. Provisional Patent Application Ser. No. 60/922,738, entitled ANANTIOXIDANT STABILIZED CROSSLINKED ULTRA-HIGH MOLECULAR WEIGHTPOLYETHYLENE FOR MEDICAL DEVICE APPLICATIONS, filed on Apr. 10, 2007,the entire disclosure of which is expressly incorporated by referenceherein.

BACKGROUND

1. Field of the Invention.

The present invention relates to crosslinked ultra-high molecular weightpolyethylene and, particularly, to antioxidant stabilized, crosslinkedultra-high molecular weight polyethylene.

2. Description of the Related Art.

Ultra-high molecular weight polyethylene (UHMWPE) is commonly utilizedin medical device applications. In order to beneficially alter thematerial properties of UHMWPE and decrease its wear rate, UHMWPE may becrosslinked. For example, UHMWPE may be subjected to electron beamirradiation, gamma irradiation, or x-ray irradiation, causing chainscissions of the individual polyethylene molecules as well as thebreaking of C-H bonds to form free radicals on the polymer chains. Whilefree radicals on adjacent polymer chains may bond together to formcrosslinked UHMWPE, some free radicals may remain in the UHMWPEfollowing irradiation, which could potentially combine with oxygen,causing oxidation of the UHMWPE.

Oxidation detrimentally affects the material properties of UHMWPE andmay also increase its wear rate. To help eliminate the free radicalsthat are formed during irradiation and that may continue to existthereafter, UHMWPE may be melt annealed by heating the crosslinkedUHMWPE to a temperature in excess of its melting point. By increasingthe temperature of the UHMWPE above its melting point, the mobility ofthe individual polyethylene molecules is significantly increased,facilitating additional crosslinking of the polyethylene molecules andthe quenching of free radicals.

While melt annealing irradiated, crosslinked UHMWPE helps to eliminatefree radicals and reduce the potential for later oxidation of theUHMWPE, the melt annealing could potentially reduce other mechanicalproperties of the UHMWPE.

SUMMARY

The present invention relates to a crosslinked UHMWPE and, particularly,an antioxidant stabilized, crosslinked UHMWPE. In one exemplaryembodiment, an antioxidant is combined with UHMWPE prior to subjectingthe UHMWPE to crosslinking irradiation. In one exemplary embodiment, theantioxidant is tocopherol. After the antioxidant is combined with theUHMWPE, the resulting blend may be formed into slabs, bar stock, and/orincorporated into a substrate, such as a metal, for example. Theresulting product may then be subjected to crosslinking irradiation. Inone exemplary embodiment, the UHMWPE blend is preheated prior tosubjecting the same to crosslinking irradiation. Once irradiated, theUHMWPE blended product may be machined, packaged, and sterilized inaccordance with conventional techniques.

In one exemplary embodiment, the formed UHMWPE/antioxidant blend may besubjected to multiple passes of crosslinking irradiation. By irradiatingthe blend in multiple passes, the maximum dose of radiation received bythe UHMWPE blend at any one time is lessened. As a result, the maximumtemperature of the UHMWPE blend reached during irradiation iscorrespondingly lessened. This allows for the UHMWPE to maintain ahigher level of desirable mechanical properties and prevents substantialmelting of the UHMWPE. In one exemplary embodiment, the UHMWPE is cooledafter each individual pass of crosslinking irradiation. By allowing theUHMWPE blend to cool, the temperature at the time of subsequentirradiation is high enough to encourage the mobility of the individualpolyethylene molecules, but is also low enough that the temperatureincrease experienced during irradiation is unlikely to substantiallyalter any desired material properties of the UHMWPE blend.

Advantageously, by incorporating an antioxidant, such as tocopherol,into the UHMWPE prior to subjecting the same to crosslinkingirradiation, the UHMWPE may be stabilized without the need for postirradiation melt annealing or any other post-irradiation treatment toquench free radicals. Specifically, an antioxidant, such as tocopherol,acts as a free radical scavenger and, in particular, acts as an electrondonor to stabilize free radicals. While tocopherol itself then becomes afree radical, tocopherol is a stable, substantially unreactive freeradical. Additionally, because of the substantially reduced level ofoxidation that occurs using a UHMWPE/antioxidant blend, the amount ofoxidized material that must be removed to form a final, implantablemedical component is reduced. As a result, the size of the stockmaterial subjected to irradiation may be smaller in dimension, making iteasier to handle and easier to manufacture into final medicalcomponents.

Moreover, by subjecting the UHMWPE/antioxidant blend to multiple passesof irradiation, the UHMWPE blend may be integrally incorporated onto asubstrate prior to irradiation. Specifically, as a result of separatingthe total radiation dose into a plurality of individual passes, thetemperature of the UHMWPE blend at the UHMWPE/substrate interfaceremains low enough that separation of the UHMWPE blend and the substrateis substantially prevented. Further, even after irradiation, someantioxidant remains unreacted within the UHMWPE blend, which maycontinue to quench free radicals throughout the lifetime of the medicalcomponent. Thus, even after the medical component is implanted, theantioxidant may continue to quench free radicals and further reduce thelikelihood of additional oxidation.

In one form thereof, the present invention provides method forprocessing UHMWPE for use in medical applications, the method includingthe steps of: combining UHMWPE with an antioxidant to form a blendhaving 0.01 to 3.0 weight percent of the antioxidant, the UHMWPE havinga melting point; processing the blend to consolidate, the consolidatedblend having a melting point; preheating the consolidated blend to apreheat temperature below the melting point of the consolidated blend;and irradiating the consolidated blend while maintaining theconsolidated blend at a temperature below the melting point of theconsolidated blend

In another form thereof, the present invention provides a crosslinkedUHMWPE blend for use in medical implants prepared by a process includingthe steps of: combining UHMWPE with an antioxidant to form a blendhaving 0.1 to 3.0 weight percent antioxidant; processing the blend toconsolidate the blend, the consolidated blend having a melting point;preheating the consolidated blend to a preheat temperature below themelting point of the consolidated blend; and irradiating theconsolidated blend with a total irradiation dose of at least 100 kGywhile maintaining the consolidated blend at a temperature below themelting point of the consolidated blend.

BRIEF DESCRIPTION OF THE DRAWING

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescription of an embodiment of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is schematic depicting exemplary processes for preparing andusing the crosslinked UHMWPE blends of the present invention; and

FIG. 2 is a perspective view of an exemplary medical implant formed forma UHMWPE blend and a substrate.

The exemplifications set out herein illustrate embodiments of theinvention and such exemplifications are not to be construed as limitingthe scope of the invention in any manner.

DETAILED DESCRIPTION

Referring to FIG. 1, UHMWPE is combined with an antioxidant to create aUHMWPE/antioxidant blend (the “UHMWPE blend”). Once combined, the UHMWPEblend may be processed to fabricate the same into a desired form. Onceformed, the UHMWPE blend may be preheated and subjected to cross-linkingirradiation. The crosslinked UHMWPE blend may then be subjected tomachining, packaging, and sterilization.

To create the UHMWPE/antioxidant blend, any medical grade UHMWPE powdermay be utilized. For example, GUR 1050 and GUR 1020 powders, bothcommercially available from Ticona, having North American headquarterslocated in Florence, Ky., may be used. Similarly, while any antioxidant,such as Vitamin C, lycopene, honey, phenolic antioxidants, amineantioxidants, hydroquinone, beta-carotene, ascorbic acid, CoQ-enzyme,and derivatives thereof, may be used, the UHMWPE blend referred toherein is a UHMWPE/tocopherol, i.e., Vitamin E, blend. Additionally, asany tocopherol may be used in conjunction with the present invention,such as d-α-tocopherol, d,1-α-tocopherol, or α-tocopherol acetate,unless otherwise specifically stated herein, the term “tocopherol” inits generic form refers to all tocopherols. However, the synthetic form,d,1-α-tocopherol, is the most commonly used.

In combining UHMWPE and tocopherol, any mechanism and/or processachieving a substantially homogenous blend of the components may beutilized. In one exemplary embodiment, solvent blending is utilized. Insolvent blending, tocopherol is mixed with a volatile solvent to lowerthe viscosity of the tocopherol and facilitate homogenous blending ofthe tocopherol with the UHMWPE. Once the tocopherol is mixed with thesolvent, the tocopherol/solvent mixture may be combined with the UHMWPE,such as with a cone mixer. The solvent is then evaporated, leaving onlythe UHMWPE/tocopherol blend. In another exemplary embodiment, tocopherolmay be blended with UHMWPE by precision coating or atomization. Forexample, tocopherol may be precision coated onto the UHMWPE powder usinga MP-1 MULTI-PROCESSOR™ Fluid Bed connected to a laboratory modulePrecision Coater available from Niro Inc. of Columbia, Md.MULTI-PROCESSOR™ is a trademark of Niro Inc.

In another exemplary embodiment, low intensity mixing may be used. Lowintensity, i.e. low shear, mixing may be performed using a Diosna P100Granulator, available from Diosna GmbH of Osnabrück, Germany, asubsidiary of Multimixing S.A. In another exemplary embodiment, highshear mixing may be used. High shear mixing of UHMWPE and tocopherol maybe achieved using a RV02E or a R05T High Intensity Mixer, bothcommercially available from Eirich Machines of Gurnee, Ill.Alternatively, high shear mixing may be achieved using a ColletteULTIMAPRO™ 75 One Pot Processor available from Niro, Inc. of Columbia,Maryland. ULTIMAPRO™ is a trademark of Niro, Inc. Based on the resultsof testing the above identified methods useful for combining UHMWPE andtocopherol, high shear mixing appears to provide favorable results,including an acceptable homogeneity and a low number of indications,i.e., areas of high tocopherol concentrations relative to thesurrounding areas as determined by visual inspection under ultravioletlight or by chemical measurements, such as infrared spectroscopy or gaschromatography. Additionally, in other exemplary embodiments, thefluidized bed, emulsion polymerization, electrostatic precipitation,wetting or coating of particles, and/or master batch blending may beused to combine the UHMWPE and tocopherol.

Irrespective of the method used to combine the UHMWPE and tocopherol toform the UHMWPE blend, the components are combined in ratios necessaryto achieve a tocopherol concentration of between 0.01 weight percent(wt. %) and 3 wt. %. In exemplary embodiments, the tocopherolconcentration may be as low as 0.01 wt. %, 0.05 wt. %, and 0.1 wt. %, oras high as 0.6 wt. %, 0.8 wt. %, and 1.0 wt. %, for example. Indetermining the appropriate amount of tocopherol, two competing concernsexist. Specifically, the amount selected must be high enough to quenchfree radicals in the UHMWPE, but must also be low enough to allowsufficient crosslinking so as to maintain acceptable wear properties ofthe UHMWPE. In one exemplary embodiment, a range of tocopherol from 0.1to 0.6 wt. % is used to successfully quench free radicals while stillmaintaining acceptable wear properties.

Once the UHMWPE blend is substantially homogenously blended and theamount of tocopherol is determined to be within an acceptable range, theUHMWPE blend is processed to consolidate the UHMWPE blend, as indicatedat Step 12 of FIG. 1. The UHMWPE blend may be processed by compressionmolding, net shape molding, injection molding, extrusion, monoblockformation, fiber, melt spinning, blow molding, solution spinning, hotisostatic pressing, high pressure crystallization, and films. In oneexemplary embodiment, as indicated at Step 16 of FIG. 1, the UHMWPEblend is compression molded into the form of a slab. In anotherexemplary embodiment, indicated at Step 14 of FIG. 1, the UHMWPE blendmay be compression molded into a substrate, as described in furtherdetail below. For example, the UHMWPE blend may be compression moldedinto a roughened surface by macroscopic mechanically interlocking theUHMWPE blend with features formed at the roughened surface of thesubstrate. Similarly, the UHMWPE blend may be molded into anotherpolymer or another antioxidant stabilized polymer. Alternatively, theUHMWPE blend may be net shape molded into the shape of the finalorthopedic component at Step 15 of FIG. 1. In this embodiment, if thefinal orthopedic component includes a substrate, the UHMWPE blend is netshape molded into the substrate at Step 15 and is processed in the samemanner as a UHMWPE blend compression molded into a substrate at Step 14,as described in detail below. In contrast, if the UHMWPE blend is netshape molded at Step 15, but is not net shape molded into a substrate,the component is then processed in the same manner as a UHMWPE blendcompression molded into a slab at Step 16, as described in detail below.

In one exemplary embodiment, the substrate may be a highly porousbiomaterial useful as a bone substitute, cell receptive material, tissuereceptive material, an osteoconductive material, and/or anosteoinductive material. A highly porous biomaterial may have a porosityas low as 55, 65, or 75 percent or as high as 80, 85, or 90 percent. Anexample of such a material is produced using Trabecular Metal™technology generally available from Zimmer, Inc., of Warsaw, Ind.Trabecular Metal™ is a trademark of Zimmer Technology, Inc. Such amaterial may be formed from a reticulated vitreous carbon foam substratewhich is infiltrated and coated with a biocompatible metal, such astantalum, etc., by a chemical vapor deposition (“CVD”) process in themanner disclosed in detail in U.S. Pat. No. 5,282,861, the entiredisclosure of which is expressly incorporated herein by reference. Inaddition to tantalum, all porous coating and other metals such asniobium, tivanium, cancellous structured titanium, or alloys of tantalumand niobium with one another or with other metals may also be used.

After processing, the UHMWPE blend may be heated to a temperature belowthe melting point of the UHMWPE blend to relieve any residual stressesthat may have been formed during processing and to provide additionaldimensional stability. In one exemplary embodiment, the melting point ofthe UHMWPE blend is determined according to standard methods usingdifferential scanning calorimetry. Heating the UHMWPE blend below themelting point creates a more homogenous mixture and increases the finalcrystallinity. In one exemplary embodiment, the UHMWPE blend is heatedto a temperature below its melting point, e.g., between 80° Celsius (C)and 140° C., and held isothermally for six hours. In other exemplaryembodiments, the UHMWPE may be heated to a temperature as low as 80° C.,90° C., 95° C., or 100° C. or as high as 110° C., 115° C., 120° C., and126° C. In other exemplary embodiments the temperature may be held foras short as 0.5 hours, 1.0 hours, 1.5 hours, or 2.0 hours or as long as3.0 hours, 4.0 hours, 5.0 hours, or 6.0 hours. In another exemplaryembodiment, the UHMWPE blend is heated after irradiation, describedbelow, to provide similar benefits to the UHMWPE blend.

Irrespective of whether the UHMWPE blend is heated to a temperaturebelow the melting point of the UHMWPE blend to relieve any residualstress, the processed UHMWPE blend is preheated at Steps 18, 20 of FIG.1 in preparation for receiving crosslinking irradiation. In oneexemplary embodiment, the processed UHMWPE blend may be preheated to anytemperature between room temperature, approximately 23° C., up to themelting point of the UHMWPE blend, approximately 140° C. In anotherexemplary embodiment, the UHMWPE blend is preheated to a temperaturebetween 60° C. and 130° C. In other exemplary embodiments, the UHMWPEblend may be heated to a temperature as low as 60° C., 70° C., 80° C.,90° C., or 100° C. or as high as 110C, 120° C., 130° C., 135° C., 140°C. By preheating the processed UHMWPE blend before irradiation, thematerial properties of the resulting irradiated UHMWPE blend areaffected. Thus, the material properties for a UHMWPE blend irradiated ata relatively cold, e.g., approximately 40° C., temperature aresubstantially different than the material properties for a UHMWPE blendirradiated at a relatively warm, e.g., approximately 120° C. toapproximately 140° C., temperature.

However, while the material properties of a UHMWPE blend irradiated at alower temperature may be superior, the wear properties, fatigueproperties, oxidation level, and free radical concentration are allnegatively affected. In contrast, while irradiation of a UHMWPE blend ata higher temperature may slightly diminish the material properties, italso results in a higher crosslinking efficiency due to higher chainmobility and adiabatic melting. Additionally, by irradiating at a highertemperature, a greater number of crosslinks are formed. Thus, there areless free radicals in the UHMWPE blend and less tocopherol is consumedby reacting with the free radicals during irradiation and immediatelythereafter. As a result, a greater amount of tocopherol remains in theblend that may react with free radicals during the UHMWPE blend'slifecycle, i.e., after irradiation. This, in turn, increases the overalloxidative stability of the UHMWPE blend.

Referring specifically to Step 18, when the UHMWPE blend and itsassociated substrate are irradiated, the substrate may rapidly increasein temperature. Thus, the temperature increase of the substrate shouldbe taken into account when determining the preheat temperature of theUHMWPE blend and substrate. In one exemplary embodiment, a UHMWPE blendformed to a highly porous substrate manufactured using Trabecular Metal™technology is preheated to a temperature between 40° C. and 120° C.prior to subjecting the substrate and UHMWPE blend to crosslinkingirradiation. A further consideration that may impact the preheattemperature is the material used to form any tooling that may contactthe UHMWPE blend and substrate during irradiation. For example, a holderused to retain the UHMWPE blend and substrate in a desired positionduring irradiation may rapidly increase in temperature at a faster ratethan the UHMWPE blend. In order to substantially eliminate this concern,the tooling should have a heat capacity substantially equal to orgreater than the heat capacity of the UHMWPE blend. In one exemplaryembodiment, the UHMWPE blend has a heat capacity substantially between1.9 J/g ° C. and 10 J/g ° C. Thus, polyether ether ketone, for example,having a heat capacity of approximately 2.8 J/g ° C., may be used toform the tooling. Alternative materials that may be used to form thetooling also include carbon fiber and other composites.

After the desired preheat temperature of the UHMWPE blend is achieved,the UHMWPE blend is subsequently irradiated at Steps 26, 28 to inducecrosslinking of the UHMWPE. Thus, as used herein, “crosslinkingirradiation” refers to exposing the UHMWPE blend to ionizing irradiationto form free radicals which may later combine to form crosslinks. Theirradiation may be performed in air at atmospheric pressure, in a vacuumchamber at a pressure substantially less then atmospheric pressure, orin an inert environment, i.e., in an argon environment, for example. Theirradiation is, in one exemplary embodiment, electron beam irradiation.In another exemplary embodiment, the irradiation is gamma irradiation.In yet another exemplary embodiment, steps 26, 28 do not requireirradiation, but instead utilize silane crosslinking. In one exemplaryembodiment, crosslinking is induced by exposing the UHMWPE blend to atotal radiation dose between about 25 kGy and 1,000 kGy. In anotherexemplary embodiment, crosslinking is induced by exposing the UHMWPEblend to a total radiation dose between about 50 kGy and 250 kGy in air.These doses are higher than doses commonly used to crosslink UHMWPE dueto the presence of tocopherol in the UHMWPE blend. Specifically, thetocopherol reacts with some of the polyethylene chains that became freeradicals during irradiation. As a result, a higher irradiation dose mustbe administered to the UHMWPE blend to achieve the same level ofcrosslinking that would occur at a lower dose in standard UHMWPE, i.e.,UHMWPE absent an antioxidant.

However, the higher irradiation dose needed to crosslink the UHMWPEblend to the same level as UHMWPE absent an antioxidant may cause agreater temperature increase in the UHMWPE blend. Thus, if the entireirradiation dose is administered to the UHMWPE blend at once, the UHMWPEblend may be heated above the melting point of the UHMWPE blend,approximately 140° C., and result in melt annealing of the UHMWPE blend.Therefore, prior to irradiating the UHMWPE blend, a determination ismade at Steps 22, 24 comparing the total crosslinking irradiation doseto be administered to the UHMWPE blend to the maximum individual dose ofradiation that can be administered to the UHMWPE blend without raisingthe temperature of the UHMWPE blend near to and/or above its meltingpoint.

Thus, if the total crosslinking irradiation dose determined in Steps 22,24 is less then the maximum individual crosslinking dose that can beadministered without raising the temperature of the UHMWPE near toand/or above the melting point, the UHMWPE is irradiated and the totalcrosslinking irradiation dose identified at Steps 22, 24 is administeredin air. In one exemplary embodiment, the maximum individual crosslinkingdose is between about 50 kGy and 1000 kGy. In one exemplary embodiment,the maximum individual crosslinking dose is 150 kGy for the UHMWPE blendalone (Step 24) and is 100 kGy for the UHMWPE blend and substratecombination (Step 22). However, the maximum individual crosslinking dosemay be any dose that does not cause the UHMWPE blend to increase intemperature above the melting point of the UHMWPE blend. Additionally,the maximum individual crosslinking dose may be dependent on the type ofirradiation used. Thus, the maximum individual crosslinking dose forelectron beam irradiation may be different than the maximum individualcrosslinking dose for gamma irradiation. In one exemplary embodiment ofthe UHMWPE blend and substrate, a heat sink may be attached to thesubstrate to dissipate heat therefrom and allow for the use of a higherindividual irradiation dose, i.e., allow for a higher dose to beadministered in a single pass. Further, in addition to the type ofirradiation used, the dose rate, temperature at which the dose isadministered, the amount of time between doses, and the level oftocopherol in the UHMWPE blend, may also affect the maximum individualcrosslinking dose.

If, at step 24, the total crosslinking dose for the UHMWPE blend isdetermined to exceed the maximum individual dose of approximately 150kGy, multiple irradiation passes are required. Similarly, if, at Step22, the total crosslinking dose for the UHMWPE blend and substrateexceeds the maximum individual dose of approximately 100 kGy, multipleirradiation passes are required. The lower maximum individual dose forthe UHMWPE blend and substrate results from the greater potentialtemperature increase of the substrate during irradiation. This potentialtemperature increase may be sufficient to melt or otherwisesignificantly alter the UHMWPE blend along the UHMWPE blend/substrateinterface. For example, as a result of the different coefficients ofthermal expansion between the UHMWPE blend and the substrate, crackingmay occur in the UHMWPE blend if irradiated at an individual dose inexcess of the maximum individual dose.

For electron beam irradiation, the preheat temperature and dose levelper pass are interdependent variables that are controlled by thespecific heat of the materials being irradiated. The substrate materialmay heat to a significantly higher level than the polymer at the sameirradiation dose level if the specific heat of the substrate issubstantially lower than the specific heat of the polymer. The finaltemperature of the materials achieved during the irradiation can becontrolled by a judicious choice of dose level per pass and preheattemperature, so that temperatures are high enough to promotecrosslinking in the presence of tocopherol, but low enough to preventsubstantial melting of the UHMWPE blend. Further, while partial meltingmay, in some embodiments, be desired, the final temperature should below enough to prevent substantial melting and yet be high enough thatfree radical levels are reduced below the levels that would be presentif no heating during irradiation had occurred. The propensity forcracking is most likely due to a combination of effects related to theweakness of the UHMWPE blend and expansion differences between theUHMWPE blend and substrate, whereas complete melting of the UHMWPE blendin the region near the substrate is due to overheating of the substrate.

If it is determined in Step 24 that multiple irradiation passes arerequired, as set forth above, then the first dose of irradiationadministered in Step 28 should be less than 150 kGy. In one exemplaryembodiment, the total irradiation dose determined in Step 24 is dividedinto equal, individual irradiation doses, each less than 150 kGy. Forexample, if the total irradiation dose determined in Step 24 is 200 kGy,individual doses of 100 kGy each may be administered. In anotherexemplary embodiment, at least two of the individual irradiation dosesare unequal and all of the individual irradiation doses do not exceeded150 kGy, e.g., a total crosslinking dose of 200 kGy is dividing into afirst individual dose of 150 kGy and a second individual dose of 50 kGy.

Similarly, if it is determined in Step 22 that multiple irradiationpasses are required, then the first dose of irradiation administered inStep 26 should be less than 100 kGy. In one exemplary embodiment, thetotal irradiation dose determined in Step 22 is divided into equal,individual irradiation doses, each less than 100 kGy. For example if thetotal irradiation dose determined in Step 22 is 150 kGy, individualdoses of 75 kGy each may be administered. In another exemplaryembodiment, at least two of the individual irradiation doses are unequaland all of the individual irradiation doses do not exceed 100 kGy, e.g.,a total crosslinking dose of 150 kGy is divided into a first individualdose of 100 kGy and a second individual dose of 50 kGy.

Further, in the UHMWPE blend/substrate embodiment, by irradiating theUHMWPE blend first, i.e., directing the electron beam to contact theUHMWPE blend prior to contacting the substrate, the resulting UHMWPEblend has characteristics similar to an irradiated UHMWPE blend withoutthe substrate and is generally suitable for normal applications. Incontrast, by irradiating the substrate first, i.e., directing theelectron beam to contact the substrate prior to contacting the UHMWPEblend, the resulting UHMWPE blend has characteristics that aresubstantially different than an irradiated UHMWPE blend without thesubstrate. Additionally, the differences, such as decreasedcrystallinity, are more pronounced near the UHMWPE blend/substrateinterface and decrease as the UHMWPE blend moves away from thesubstrate.

In the event multiple irradiation passes are required as described indetail above, the temperature of the UHMWPE blend or UHMWPE blend andsubstrate may be equilibrated to the preheat temperature in Steps 30,32, between the administration of the individual doses. Specifically, asa result of the first individual irradiation dose increasing thetemperature of the UHMWPE blend, immediately administering anotherindividual irradiation dose may significantly alter the materialproperties of the UHMWPE blend, melt the UHMWPE, or cause otherdetrimental effects. In one exemplary embodiment, after the firstindividual irradiation dose is administered, the UHMWPE blend or UHMWPEblend and substrate are removed and placed in an oven. The oven is setto maintain the temperature at the preheat temperature, i.e., thetemperature used in Steps 18, 20 as described in detail above, and theUHMWPE blend or UHMWPE blend and substrate are placed within the oven toslowly cool until reaching the preheat temperature. Once the preheattemperature is reached, the UHMWPE blend or UHMWPE blend and substrateare removed and the next individual irradiation dose administered. Inthe event further individual irradiation doses are required, thetemperature equilibration process is repeated.

In another exemplary embodiment, selective shielding is used to protectcertain areas of the UHMWPE blend from exposure to the irradiation andsubstantially prevent or lessen the resulting temperature increase ofthe UHMWPE blend. Additionally, selective shielding may be used to helpensure that an even dose of irradiation is received by the UHMWPE blend.In one embodiment, a shield, such as a metallic shield, is placed in thepath of the irradiation to attenuate the radiation dose received in theshielded area, while allowing the full effect of the irradiation dose inareas where higher temperatures can be tolerated. In one embodiment, theuse of selective shielding allows for the total crosslinking irradiationdose to be administered in a single pass, reducing the need toadminister the total crosslinking irradiation dose over multiple passes.

Additionally, selective shielding of the irradiation may be used toprevent the metallic substrate from excessive heating due to thedifferences in specific heats between the substrate and UHMWPE blend.The shielding could, in one exemplary embodiment, be designed so thatthe UHMWPE blend receives a substantially full irradiation dose, whilelessening the irradiation penetration so that a reduced dose is receivedat the substrate. As a result, the temperature increase of the substratedue to irradiation absorption is decreased. This allows for the use ofhigher dose levels per pass, eliminating the need for multiple passes toachieve higher dose levels and thus higher levels of crosslinking. Insome embodiments, single pass irradiation is advantageous since it is amore efficient manufacturing process, and the resulting mechanicalproperties of the crosslinked material may also be desirable. Specificaspects and methods of irradiation shielding are disclosed in U.S. Pat.No. 6,365,089, entitled METHOD FOR CROSSLINKING UHMWPE IN AN ORTHOPEDICIMPLANT, issued on Apr. 2, 2002, the entire disclosure of which isexpressly incorporated by reference herein.

In another exemplary embodiment, tocopherol is not added at Step 10, asdiscussed in detail above. Instead, tocopherol is diffused into theUHMWPE by placing the UHMWPE in a tocopherol bath after the UHMWPE hasbeen irradiated in accordance with standard crosslinking irradiationtechniques. However, as a result of administering the crosslinkingirradiation prior to the addition of tocopherol, the present embodimentdoes not allow for the administration of a higher crosslinkingirradiation dose, as discussed above. Additionally, the mechanicalproperties achieved by adding tocopherol prior to administeringcrosslinking irradiation appear to be superior to diffusing tocopherolinto the UHMWPE after the crosslinking irradiation has beenadministered.

Once the total crosslinking irradiation dose has been administered tothe UHMWPE blend, the UHMWPE blend may be machined in Step 34 into amedical product, such as an orthopedic implant, according to customarytechniques, such as milling, boring, drilling, cutting, and CNC(Computer Numerical Control) machining. For example, the UHMWPE blendmay be machined into a hip, knee, ankle, shoulder, elbow, finger,dental, or spinal implant. Additionally, the UHMWPE blend may beassembled to other components for form a medical device. However, if theUHMWPE blend is processed at Step 12 in FIG. 1 by net shape molding,which is identified above as a potential processing method, the need tomachine the UHMWPE blend at Step 34 is substantially eliminated.Specifically, if the UHMWPE blend is processed by net shape molding, theUHMWPE blend is formed to the final shape, i.e., the shape of thedesired medical product, at Step 12, which may then be assembled toother components to form the final medical device. Referring to FIG. 2,an exemplary medical implant 100 is shown including UHMWPE blend 102 andsubstrate 104. As shown in FIG. 2, UHMWPE blend 102 is interdigitatedwith substrate 104 in a similar manner as described in U.S. patentapplication Ser. No. 11/055,322, entitled “MODULAR POROUS IMPLANT”,filed Feb. 10, 2002, and U.S. Pat. No. 6,087,553, entitled “IMPLANTABLEMETALLIC OPEN-CELLED LATTICE/POLYETHYLENE COMPOSITE MATERIAL ANDDEVICES”, issued on Jul. 11, 2000, the entire disclosures of which areexpressly incorporated by reference herein.

The medical product may be packaged at Step 36 and sterilized at Step38. In one exemplary embodiment, the medical product is sterilized usinggas plasma. In another exemplary embodiment, the medical product issterilized using ethylene oxide. In yet another exemplary embodiment,the medical product is sterilized using dry heat sterilization.Additionally, testing has indicated that surface sterilizationtechniques, such as gas plasma, dry heat, gamma radiation, ionizingradiation, autoclaving, supercritical fluid technique, and ethyleneoxide, provide sufficient sterilization of the medical product, even ifthe UHMWPE blend is secured to a substrate. Specifically, the surfacesterilization techniques have proven to sufficiently sterilize theUHMWPE blend/substrate interface. In another exemplary embodiment, gammairradiation may be used to sterilize the medical product. However, inthis embodiment, it is believed that a higher tocopherol concentrationwould be necessary in order for enough tocopherol to be available toquench free radicals after the sterilization irradiation was performed.

While this invention has been described as having a preferred design,the present invention can be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

EXAMPLES

The following non-limiting Examples illustrate various features andcharacteristics of the present invention, which is not to be construedas limited thereto. The following abbreviations are used throughout theExamples unless otherwise indicated.

TABLE 1 Abbreviations Abbreviation Full Word kGy kilo Gray min minuteMeV mega electron volt m meter ° degrees C. Celsius FTIR FourierTransform Infrared Spectroscopy wt. % weight percent MPa mega pascal UTSultimate tensile strength UHMWPE ultrahigh molecular weight polyethyleneYS yield strength HXPE highly crosslinked polyethylene OI OxidationIndex T Temperature DSC Differential Scanning Calorimetry ml milliliternm nanometer TVI trans-vinylene index VEI d/l-α-tocopherol index Mcmillion cycles molecular weight between crosslinks AVE aged vitamin Epercent AVEI aged vitamin E index AV-OI aged oxidation index mgmilligram cm centimeter IR infrared VE % weight percent tocopherol Volvolume wt. weight VE tocopherol g gram DMA Dynamic Mechanical AnalysiskJ kilojoule Izod Izod Impact Strength Conc. Concentration dm decimeter

Throughout the various Examples, irradiated UHMWPE blends are used,which have been irradiated according to one of three differentirradiation methods. As set forth above, differences in the irradiationconditions and techniques may affect the resulting material propertiesof the UHMWPE blend. Therefore, in order to properly analyze and comparethe results set forth in the Examples and corresponding Tables, each ofthe irradiated UHMWPE blends used in the Examples are identified ashaving been irradiated according to the one of the methods set forthbelow in Table 2. Additionally, the electron beam source is calibratedby performing dosimetry at low irradiation doses and then parametricallydetermining the activation of the electron beam source needed to achievehigher doses. As a result, at higher irradiation doses, differences mayexist between the actual dose and the parametrically determined dose,which may cause differences in the material properties of the irradiatedUHMWPE blends.

TABLE 2 Irradiation Methods Method A Method B Method C Dose Rate (kGy-30-75 16-25 75-240 m/min) Dose Level (kGy) 160-190 133-217 90-200Electron Beam 10 12 10 Energy (MeV) Method of Water AluminumRadiochromic Dosimetry Calorimeter Calorimeter Film

EXAMPLE 1 Feasibility Study of α-Tocopherol Acetate

The feasibility of blending α-tocopherol acetate with UHMWPE wasinvestigated. α-tocopherol acetate was obtained from DSM NutritionalProducts, Ltd. of Geleen, Netherlands and medical grade UHMWPE powderGUR 1050 was obtained from Ticona, having North American headquarterslocated in Florence, Ky. Isopropanol was then added to the α-tocopherolacetate as a diluent and the α-tocopherol acetate was solvent blendedwith the UHMWPE powder. The blending continued until two differentUHMWPE/α-tocopherol acetate blends were obtained, one UHMWPE blendhaving 0.05 wt. % α-tocopherol acetate and the other UHMWPE blend having0.5 wt. % α-tocopherol acetate. Each of the UHMWPE blends were thencompression molded to form four one-inch-thick pucks. Two pucks of eachUHMWPE blend, i.e., two pucks of the UHMWPE blend having 0.05 wt. %α-tocopherol acetate and two pucks of the UHMWPE blend having 0.5 wt. %α-tocopherol acetate, were preheated to 120° C. in a Grieve convectionoven, available from The Grieve Corporation of Round Lake, Ill. Thepucks were held at 120° C. for 8 hours. After the expiration of 8 hours,the pucks were irradiated at 10 MeV, 50 kGy-m/min dose rate at 65 kGyand 100 kGy dose at Iotron Industries Canada Inc. located in PortCoquitlam, BC, Canada.

The remaining two pucks of each UHMWPE blend, i.e., two pucks of theUHMWPE blend having 0.05 wt. % α-tocopherol acetate and two pucks of theUHMWPE blend having 0.5 wt. % α-tocopherol acetate, were heated to 40°C. overnight. The next morning, the remaining two pucks of each UHMWPEblend were irradiated at 10 MeV, 50 kGy-m/min dose rate at 100 kGy doseat Iotron Industries Canada Inc. located in Port Coquitlam, BC, Canada.

After irradiation, all of the pucks were cut in half and a film was cutfrom the center of each puck. The films were then subjected to FTIRanalysis using a Bruker Optics FTIR Spectrometer, available from BrukerOptics of Billerica, Mass. Both halves of each puck were then machinedinto flat sheets approximately ⅛ inch thick. One half of the flat sheetswere immediately subjected to FTIR. The other half of the flat sheetswere then subjected to accelerated aging in accordance with the AmericanSociety for Testing and Materials (ASTM) Standard F-2003, StandardPractice for Accelerated Aging of Ultra-High Molecular WeightPolyethylene after Gamma Irradiation in Air. Tensile specimens formedfrom the flat sheets were subjected to accelerated aging and were thensubjected to FTIR analysis. The OI and wt. % of α-tocopherol acetatewere determined from the FTIR results, set forth below in TABLES 3 and4. However, there were interference peaks in the FTIR results thatprevented measurement of OI for the 0.5 wt. %, 65 kGy, unaged sample.

TABLE 3 FTIR Results wt. % wt. % Dose, tocopherol, tocopherol kGyCondition OI meas. 0.50 100 Un-aged <0/<0 0.17/0.15 Aged 0.0323 0.150.50 65 Un-aged *interference* 0.14/0.15 Aged 0.0083 0.14 0.05 100Un-aged 0.0300/0.0948 0.01/0.00 Aged 0.0647 <0     0.05 65 Un-aged0.0376/0.0940 0.02/0.00 Aged 0.0647 <0    

TABLE 4 Oxidative Index of UHMWPE Blend with 0.50 Weight Percentα-Tocopherol Acetate

The FTIR results revealed that the OI of the UHMWPE blend having 0.05wt. % α-tocopherol acetate was generally higher than the OI of theUHMWPE blend having 0.50 wt. % α-tocopherol acetate. This is believed tobe because these samples still contained α-tocopherol acetate afterirradiation. As a result, the α-tocopherol acetate was still availablein these samples to react with free radicals and reduce the oxidativedegradation of the UHMWPE blend. Additionally, the FTIR results showedthat virtually no α-tocopherol acetate was left after irradiation of theUHMWPE blend having 0.05 wt. % α-tocopherol acetate and that aboutone-third of the α-tocopherol acetate was left after irradiation of theUHMWPE blend having 0.5 wt. % α-tocopherol acetate. Further, as shown inTABLE 5 below, tensile properties were similar for both the UHMWPEblends that were subjected to accelerated aging and the UHMWPE blendsthat were not subjected to accelerated aging. Finally, the FTIR resultssuggested that the UHMWPE blends containing α-tocopherol acetate havesimilar stabilization properties, i.e., a similar ability to preventoxidative degeneration, as UHMWPE blends containing similarconcentration of d,1-α-tocopherol.

TABLE 5 Mechanical Properties wt. % α- tocopherol Dose, kGy Yield, UTS,acetate (Temperature) Condition Elongation, % MPa MPa 0.50 100 (40° C.)Un-aged 356.7 23.1 61.7 Aged 360.1 25.3 65.3 0.50  65 (120° C.) Un-aged384.6 21.8 61.4 Aged 378.4 24.2 65.1 0.05 100 (40° C.) Un-aged 342.921.5 56.7 Aged 288.6 25.2 61.0 0.05  65 (120° C.) Un-aged 352.4 21.457.4 Aged 287.7 25.2 59.6

EXAMPLE 2 Chemical Properties of UHMWPE Blended with Tocopherol

The chemical properties of d/1-α-tocopherol mechanically blended with aUHMWPE powder which was slab molded into bars and electron beamirradiated were investigated. To perform this investigation, DesignExpert 6.0.10 software, obtained from Stat-Ease, Inc. Minneapolis,Minn., was utilized to setup a modified fractional factorial Design ofExperiment (DOE). The DOE evaluated five different variables: UHMWPEresin type, wt. % of d/1-α-tocopherol, preheat temperature, dose rate,and irradiation dose.

GUR 1050 and GUR 1020 medical grade UHMWPE powders were obtained fromTicona, having North American headquarters in Florence, Ky.d/1-α-tocopherol was obtained from DSM Nutritional Products, Ltd. ofGeleen, Netherlands. The GUR 1050 and GUR 1020 were separatelymechanically blended with the d/1-α-tocopherol by low intensity blendingusing a Diosna P100 Granulator, available from Diosna GmbH of Osnabrück,Germany, a subsidiary of Multimixing S.A. Both the GUR 1050 and the GUR1020 resins were mixed with the d/1-α-tocopherol in several batches tocreate UHMWPE blends of both resin types having 0.2 wt. %, 0.5 wt. %,and 1.0 wt. % d/1-α-tocopherol. Each batch of blended material wascompression molded into a slab and cut into bars of various sizes. Eachof the resulting bars was then preheated by heating to a preheattemperature in a Grieve convection oven, available from The GrieveCorporation of Round Lake, Ill. The preheat temperature was selectedfrom 40° C., 100° C., 110° C. and 122.2° C., as set forth in TABLE 6below.

After being preheated, the UHMWPE blend bars were electron beamirradiated according to Method C, set forth in TABLE 2 above, at aselected dose rate until a selected total irradiation dose wasadministered. The dose rate was selected from 75 kGy-m/min, 155kGy-m/min, and 240 kGy-m/min and the total irradiation dose was selectedfrom 90 kGy, 120 kGy, 150 kGy, and 200 kGy. The portion of each bar wasthen microtomed into 200 micron thick films. These films were thensubjected to FTIR analysis on a Bruker Optics FTIR spectrometer,available from Bruker Optics of Billerica, Mass. The FTIR results wereanalyzed to determine the VEI, wt. % d/1-α-tocopherol, the OI, and theTVI. The VEI and wt. % d/1-α-tocopherol were determined by calculatingthe ratio of the area under the d/1-α-tocopherol peak at 1275-1245 cm⁻¹on the resulting FTIR chart to the area under the polyethylene peak at1392-1330 cm⁻¹ and at 1985-1850 cm⁻¹. The OI was determined bycalculating the ratio of the area under the carbonyl peak on the FTIRchart at 1765-1680cm⁻¹ to the area of the polyethylene peak at 1392-1330cm⁻¹. The TVI was determined by calculating the ratio of the area on theFTIR chart under the vinyl peak at 980-947 cm⁻¹ to the area under thepolyethylene peak at 1392-1330 cm⁻¹.

After the initial VEI, wt. % d/1-α-tocopherol and TVI were determinedfrom the FTIR analysis of the thin films, each of the thin films wereaccelerated aged according to ASTM Standard F-2003, Standard Practicefor Accelerated Aging of Ultra-High Molecular Weight Polyethylene afterGamma Irradiation in Air. The accelerated aged films were againsubjected to FTIR analysis on a Bruker Optics FTIR spectrometer,available from Bruker Optics of Billerica, Mass. The resulting FTIRcharts were analyzed to determine VEI, wt. % d/1-α-tocopherol, OI, andTVI according to the methods set forth above. Once subjected to FTIRanalysis, the aged files were placed in boiling hexane and allowed toremain there for 24 hours to extract the d/1-α-tocopherol. Afterextraction of the d/1-α-tocopherol, the aged films were again subjectedto FTIR analysis on the Bruker Optics FTIR spectrometer. The resultingFTIR chart was then analyzed to determine the OI in accordance with themethod set forth above. The additional FTIR analysis was performed toeliminate the d/1-α-tocopherol peak from interfering with the oxidationpeaks. An analysis of the results set forth in TABLE 6 below indicatethat selecting a warmer preheat temperature may result in a lower OI andmay also result in some of the d/1-α-tocopherol remaining in the UHMWPEafter irradiation.

TABLE 6 FTIR Results of Irradiated UHMWPE Blended with d/l-α-tocopherolPre-heat Dose VE level Dose Rate Resin Run (° C.) (kGy) (° C.)(kGy-m/min) Type (GUR) 1 122 150 1 75 1020 2 40 200 0.2 155 1020 3 12290 0.5 75 1020 4 122 200 0.2 155 1020 5 40 90 0.2 240 1050 6 122 90 0.275 1050 7 40 150 0.2 75 1050 8 122 150 0.2 75 1020 9 40 90 1 240 1050 1040 200 0.5 155 1020 11 122 90 0.2 240 1020 12 40 90 1 75 1020 13 40 1500.5 75 1020 14 122 150 0.2 240 1050 15 122 150 0.5 240 1020 16 40 1500.2 240 1020 17 40 90 0.2 75 1020 18 122 200 0.5 155 1020 19 122 90 1240 1020 20 40 150 1 240 1020 21 40 200 1 155 1020 22 122 200 1 155 102023 40 200 0.2 155 1050 24 122 200 0.2 155 1050 25 40 200 0.5 155 1050 26122 200 0.5 155 1050 27 40 200 1 155 1050 28 122 200 1 155 1050 29 40120 0.5 157.5 1050 30 122 120 0.5 157.5 1050 31 40 120 1 157.5 1050 32122 120 1 157.5 1050 33 40 90 1 75 1050 34 122 90 0.5 75 1050 35 40 1500.5 75 1050 36 122 150 1 75 1050 37 40 90 0.5 240 1050 38 122 90 1 2401050 39 40 150 1 240 1050 40 122 150 0.5 240 1050 VE % VE Index (aged)VE % VE % VE Index 1900 nm 1370 nm 1370 nm IR 1900 nm IR 1370 nm IR IRIR Run peak peak peak peak peak 1 0.803 0.682 0.046 0.171 0.493 2 0.0400.048 0.004 0.015 0.022 3 0.359 0.321 0.021 0.082 0.248 4 0.045 0.0540.004 0.016 0.037 5 0.047 0.055 0.004 0.016 0.063 6 0.061 0.071 0.0050.020 0.073 7 0.011 0.025 0.003 0.009 0.017 8 0.031 0.042 0.004 0.0130.033 9 0.194 0.165 0.012 0.044 0.272 10 0.731 0.626 0.042 0.157 0.54511 0.075 0.078 0.006 0.022 0.081 12 0.882 0.738 0.050 0.185 0.417 130.286 0.222 0.017 0.058 0.274 14 0.058 0.072 0.005 0.021 0.056 15 0.1620.151 0.011 0.040 0.279 16 0.051 0.053 0.005 0.016 0.050 17 0.078 0.0760.006 0.022 0.044 18 0.721 0.634 0.041 0.159 0.524 19 0.769 0.688 0.0440.173 0.430 20 0.781 0.597 0.044 0.150 0.531 21 0.769 0.591 0.044 0.1490.560 22 0.765 0.607 0.044 0.153 0.575 23 0.028 0.034 0.003 0.011 0.01624 0.051 0.053 0.005 0.016 0.041 25 0.288 0.249 0.018 0.064 0.281 260.320 0.282 0.019 0.073 0.309 27 0.284 0.222 0.017 0.058 0.281 28 0.3080.241 0.019 0.062 0.295 29 0.613 0.550 0.035 0.139 0.489 30 0.753 0.7000.043 0.176 0.445 31 0.283 0.240 0.017 0.062 0.279 32 0.306 0.288 0.0190.074 0.259 33 0.779 0.706 0.044 0.177 0.429 34 0.328 0.314 0.020 0.0800.209 35 0.143 0.125 0.010 0.034 0.247 36 0.803 0.758 0.046 0.190 0.44237 0.332 0.291 0.020 0.075 0.262 38 0.741 0.731 0.042 0.183 0.390 390.790 0.658 0.045 0.165 0.524 40 0.327 0.301 0.020 0.077 0.282 VE IndexVE % VE Index (aged) OI (aged) (aged) 1900 nm (Extraction- TVI 1900 nm1370 nm IR aged) TVI (aged) Run IR peak IR peak peak FTIR FTIR FTIR 10.425 0.029 0.108 −0.010 0.061 0.062 2 0.033 0.003 0.011 0.078 0.0730.072 3 0.226 0.015 0.059 −0.008 0.046 0.052 4 0.048 0.004 0.015 0.0250.081 0.082 5 0.068 0.005 0.020 0.039 0.039 0.040 6 0.080 0.006 0.0230.000 0.054 0.048 7 0.029 0.003 0.010 0.086 0.068 0.064 8 0.042 0.0040.013 0.035 0.076 0.074 9 0.231 0.017 0.060 0.011 0.048 0.040 10 0.4680.032 0.118 0.005 0.076 0.077 11 0.084 0.006 0.024 0.007 0.055 0.054 120.359 0.025 0.091 −0.001 0.034 0.035 13 0.211 0.017 0.055 0.082 0.0750.074 14 0.070 0.005 0.020 0.001 0.072 0.072 15 0.244 0.017 0.063 0.0250.081 0.084 16 0.052 0.005 0.016 0.075 0.063 0.062 17 0.048 0.004 0.0150.098 0.045 0.045 18 0.458 0.030 0.116 −0.004 0.083 0.085 19 0.392 0.0250.100 −0.010 0.041 0.047 20 0.409 0.031 0.104 0.087 0.080 0.078 21 0.4290.032 0.109 0.061 0.081 0.087 22 0.457 0.033 0.116 −0.007 0.092 0.091 230.025 0.003 0.009 0.120 0.078 0.079 24 0.045 0.004 0.014 0.032 0.0840.085 25 0.241 0.017 0.062 0.009 0.075 0.073 26 0.268 0.019 0.069 −0.0020.085 0.083 27 0.220 0.017 0.057 0.024 0.080 0.079 28 0.229 0.018 0.0590.042 0.094 0.096 29 0.429 0.028 0.109 0.040 0.053 0.058 30 0.421 0.0260.107 0.000 0.063 0.064 31 0.236 0.017 0.061 0.063 0.061 0.061 32 0.2440.016 0.063 0.004 0.065 0.066 33 0.397 0.025 0.101 0.036 0.040 0.041 340.205 0.013 0.053 −0.005 0.049 0.051 35 0.211 0.015 0.055 0.067 0.0720.068 36 0.423 0.026 0.107 −0.012 0.055 0.058 37 0.234 0.016 0.061 0.0290.041 0.048 38 0.391 0.023 0.099 −0.004 0.038 0.042 39 0.440 0.030 0.1110.043 0.073 0.076 40 0.262 0.017 0.068 −0.004 0.068 0.068

EXAMPLE 3 Free Radical Concentrations in UHMWPE Blended withd/1-α-Tocopherol

The impact of mechanically blending d/1-α-tocopherol with UHMWPE powderon free radical concentration of electron beam irradiated UHMWPE blendmolded pucks was investigated. To perform this investigation, DesignExpert 6.0.10 software, obtained from Stat-Ease, Inc. Minneapolis,Minn., was utilized to setup a modified central composite Design ofExperiment (DOE). The DOE evaluated five factors: preheat temperature,dose rate, irradiation dose, d/1-α-tocopherol concentration, andpredetermined hold time, i.e., the time elapsed between removal of theUHMWPE blend from the oven until the initiation of electron beamirradiation.

GUR 1050 medical grade UHMWPE powder was obtained from Ticona, havingNorth American headquarters in Florence, Ky. d/1-α-tocopherol wasobtained from DSM Nutritional Products, Ltd. of Geleen, Netherlands. TheGUR 1050 UHMWPE power was mechanically blended with the d/1-α-tocopherolby high intensity blending using an Eirich Mixer, available from EirichMachines, Inc. of Gurnee, Ill. The GUR 1050 resin was mixed with thed/1-α-tocopherol in several batches to create UHMWPE blends havingbetween 0.14 and 0.24 wt. % d/1-α-tocopherol, as set forth below inTABLE 7.

Each of the UHMWPE blends were then compression molded into 2.5 inchdiameter and 1 inch thick pucks. Each of the resulting pucks was thenpreheated by heating in a Grieve convection oven, available from TheGrieve Corporation of Round Lake, Ill., to a preheat temperature. Thepreheat temperature was selected from between 85° C. and 115° C., as setforth in TABLE 7 below. The pucks were then removed from the convectionoven and held for a predetermined period of time ranging between 7minutes and 21 minutes, as set forth in TABLE 7 below. After theexpiration of the predetermined hold time, the pucks were electron beamirradiated utilizing Method A of TABLE 2. The pucks were irradiated at adose rate selected from between 30 kGy-m/min and 75 kGy-m/min until atotal dose selected from between 160 kGy and 190 kGy was administered,as set forth in TABLE 7 below. Cylindrical cores approximately 1 inchlong were machined from the pucks. The cylindrical cores were thenanalyzed using a Bruker EMX/EPR (electron paramagnetic resonance)spectrometer, which has a detection limit of 0.01×10¹⁵ spins/gram and isavailable from Bruker Optics of Billerica, Mass. The resulting analysisindicated that preheat temperature, percent d/1-α-tocopherol, and doselevel were all significant factors in determining the resulting freeradical concentration of the UHMWPE blend. Specifically, preheattemperature and d/1-α-tocopherol concentration had a negativecorrelation with the free radical concentration, while the total dosehad a positive correlation with the free radical concentration.

TABLE 7 Free Radical Concentration of UHMWPE Blends After VariousProcessing Free Dose radicals Rate Oven to (spins/ Preheat Dose (kGy-Beam gram × Run Block (° C.) (kGy) VE % m/min.) (minutes) E10-16) 1Block 1 85 190 0.11 30 7 2.87 2 Block 1 115 190 0.11 30 7 1.09 3 Block 1115 190 0.11 30 21 2.01 4 Block 1 85 190 0.11 30 21 3.7 5 Block 1 115160 0.11 30 7 1.09 6 Block 1 85 160 0.11 30 7 2.55 7 Block 1 115 1600.11 30 21 1.5 8 Block 1 85 160 0.11 30 21 2.77 9 Block 1 85 160 0.22 757 2.37 10 Block 1 115 160 0.22 75 7 0.826 11 Block 1 85 160 0.22 75 212.46 12 Block 1 115 160 0.22 75 21 1.38 13 Block 1 115 190 0.22 75 70.786 14 Block 1 85 190 0.22 75 7 3.22 15 Block 1 115 190 0.22 75 211.28 16 Block 1 85 190 0.22 75 21 2.94 17 Block 1 100 175 0.165 52.5 142.46 18 Block 1 100 175 0.165 52.5 14 2.66 19 Block 1 100 175 0.165 52.514 2.98 20 Block 1 100 175 0.165 52.5 14 3.03

EXAMPLE 4 Mechanical Properties of UHMWPE Blended with d/1-α-Tocopherol

The mechanical properties of d/1-α-tocopherol mechanically blended witha UHMWPE powder which was slab molded into bars and electron beamirradiated were investigated. To perform this investigation, DesignExpert 6.0.10 software, obtained from Stat-Ease, Inc. Minneapolis,Minn., was utilized to setup a modified fractional factorial Design ofExperiment (DOE). The DOE evaluated five different variables: UHMWPEresin type, weight percent of d/1-α-tocopherol, preheat temperature,dose rate, and irradiation dose.

GUR 1050 and GUR 1020 medical grade UHMWPE powders were obtained fromTicona, having North American headquarters in Florence, Ky.d/1-α-tocopherol was obtained from DSM Nutritional Products, Ltd. ofGeleen, Netherlands. The GUR 1050 and GUR 1020 were separatelymechanically blended with the d/1-α-tocopherol by low intensity blendingusing a Diosna P100 Granulator, available from Diosna GmbH of Osnabrück,Germany, a subsidiary of Multimixing S.A. Both the GUR 1050 and the GUR1020 resins were mixed with the d/1-α-tocopherol in several batches tocreate UHMWPE blends of both resin types having 0.2 wt. %, 0.5 wt. %,and 1.0 wt. % d/1-α-tocopherol. Each batch of blended material wascompression molded into a slab and cut into bars. Each of the resultingbars was then preheated by heating the bars in a Grieve convection oven,available from The Grieve Corporation of Round Lake, Ill., to a preheattemperature. The preheat temperature was selected from 40° C., 100° C.,110° C. and 122.2° C., as set forth in TABLE 8 below.

After being preheated, the UHMWPE blend bars were electron beamirradiated according to Method C, set forth in TABLE 2 above, at aselected dose rate until a selected total irradiation dose wasadministered. The dose rate was selected from 75 kGy-m/min, 155kGy-m/min, and 240 kGy-m/min and the total irradiation dose was selectedfrom 90 kGy, 120 kGy, 150 kGy, 200 kGy, and 250 kGy. Type V tensilespecimens, as defined by the American Society for Testing and Materials(ASTM) Standard D638, Standard Test Method for Tensile Properties ofPlastics, were machined from each of the UHMWPE blend bars. The Type Vtensile specimens were then subjected to ultimate tensile elongation,UTS, and YS testing in accordance with ASTM Standard D638. Izodspecimens were also machined from each of the UHMWPE blend bars andtested for izod impact strength according to ASTM Standard D256,Standard Test Methods for Determining the Izod Pendulum ImpactResistance of Plastics. Dynamic mechanical analysis (DMA) specimens werealso machined from each of the UHMWPE blend bars and tested using aModel DMA 2980 Dynamic Mechanical Analyzer from TA Instruments of NewCastle, Del.

An analysis of the results indicates that the total irradiation dose hadan influence on the izod impact strength, ultimate tensile elongation,and yield strength of the UHMWPE blends. Additionally, the preheattemperature had an influence on the ultimate tensile strength and yieldstrength. In contrast, the weight percent of d/1-α-tocopherol had aninfluence on ultimate tensile elongation and the dynamic mechanicalanalysis. Additional results from the testing are set forth below inTABLE 8.

TABLE 8 Mechanical Properties of UHMWPE Blended with d/l-α-tocopherolPreheat Dose VE Dose Rate Izod UTS YS DMA Std ° C. kGy Conc. kGy-m/minResin kJ/m{circumflex over ( )}2 Elongation % MPa MPa MPa 1 40 90 0.2 751020 90.79 Not Tested 5.45 2 122.2 90 0.2 75 1050 74.8 348.8 50.38 21.726.12 3 40 150 0.2 75 1050 59.66 300.9 56.2 25.1 6.78 4 100 150 0.2 751020 66.05 314.8 52.42 25.24 5.76 5 40 90 1 75 1020 111.19 Not Tested4.36 6 122.2 90 0.5 75 1020 91.55 Not Tested 5.1 7 40 150 0.5 75 102079.96 355.7 55.83 26.28 4.96 8 100 150 1 75 1020 81.25 Not Tested 4.78 940 90 0.2 240 1050 82.01 319.3 56.83 23.06 6.18 10 122.2 90 0.2 240 102084.5 Not Tested 5.43 11 40 150 0.2 240 1020 67.53 293.1 56.19 26.87 5.9612 100 150 0.2 240 1050 67.75 307.2 52.95 25.21 6.31 13 40 90 1 240 1050106.17 411.2 61.76 24.89 4.53 14 122.2 90 1 240 1020 94.66 Not Tested4.83 15 40 150 1 240 1020 93.79 Not Tested 5.6 16 100 150 0.5 240 102073.08 342.2 51.54 23.39 5.22 17 40 120 0.5 157.5 1050 99.87 374.6 57.9623.23 5.06 18 110 120 0.5 157.5 1050 90.67 363.6 50.97 22.15 5.42 19 40120 1 157.5 1050 94.34 352.5 58.5 23.64 5.53 20 110 120 1 157.5 105085.01 344.9 48.98 21.95 5.72 21 40 90 1 75 1050 107.07 396.4 61.25 23.135.02 22 122.2 90 0.5 75 1050 93.44 375.8 51.47 21.92 5.7 23 40 150 0.575 1050 82.09 330.4 56.65 25.78 4.62 24 100 150 1 75 1050 88.28 NotTested 5.4 25 40 90 0.5 240 1050 102.39 36.9 58.4 23.31 5.16 26 122.2 901 240 1050 96.6 381.9 50.3 21.29 5.44 27 40 150 1 240 1050 89.5 NotTested 5.16 28 100 150 0.5 240 1050 78.51 332.9 50.18 22.08 5.6 29 40200 0.2 155 1020 55.98 246.2 52.37 27.23 6.32 30 110 200 0.2 155 102052.98 268.4 48.28 24.82 6.05 31 40 200 0.5 155 1020 74.64 310.7 53.5325.42 5.38 32 110 200 0.5 155 1020 65.47 309.1 49.13 24.21 5.31 33 40200 1 155 1020 72.67 362.9 55.62 25.9 4.63 34 110 200 1 155 1020 66.62349.7 50.45 24.24 4.93 35 40 200 0.2 155 1050 57.82 226.4 50.92 25.37.15 36 110 200 0.2 155 1050 59.04 259.4 46.4 23.7 6.57 37 40 200 0.5155 1050 67.49 280.6 51.91 26.23 5.88 38 110 200 0.5 155 1050 64.6 304.851.23 25.14 5.7 39 40 200 1 155 1050 82.01 328.9 53.79 24.43 5.15 40 110200 1 155 1050 69.42 329.7 49.54 23.12 5.27 41 100 150 0.2 240 1050 NotTested 307.2 52.96 23.27 Not Tested 42 100 150 0.2 240 1020 Not Tested288.6 49.28 23.35 Not Tested

EXAMPLE 5 Wear Properties of UHMWPE Mixed with d,1-α-tocopherol

The wear properties of UHMWPE mechanically blended with d,1-α-tocopheroland exposed to electron beam irradiation was investigated. To performthis investigation, Design Expert 6.0.10 software, obtained fromStat-Ease, Inc. Minneapolis, Minn., was utilized to setup a modifiedcentral composite Design of Experiment (DOE). The DOE evaluated fivedifferent variables: preheat temperature, dose rate, total doseadministered, d,1-α-tocopherol concentration, and cooling period, i.e.,the elapsed time from end of the preheat until initial exposure toirradiation.

GUR 1050 medical grade UHMWPE powder was obtained from Ticona, havingNorth American headquarters in Florence, Ky. d/1-α-tocopherol wasobtained from DSM Nutritional Products, Ltd of Geleen, Netherlands. TheGUR 1050 was mechanically mixed with the d/1-α-tocopherol using a HighIntensity Mixer, available from Eirich Machines of Gurnee, Ill. The GUR1050 resin was mixed with the d/1-α-tocopherol in several batches tocreate UHMWPE blends having a selected wt. % of d/1-α-tocopherol. Thewt. % of d/1-α-tocopherol was selected from 0.14 wt. %, 0.19 wt. %, and0.24 wt. % d/1-α-tocopherol. Each of the blends were then consolidatedand formed into 2.5 inch diameter and 1 inch thick pucks. Each of theresulting pucks was then preheated by heating the pucks in a Grieveconvection oven, available from The Grieve Corporation of Round Lake,Ill., to a preheat temperature. The preheat temperature was selectedfrom 85° C., 100° C., and 115° C., as set forth in TABLE 9 below.

After being preheated, the UHMWPE blend pucks were then removed from theconvection oven for a cooling period. The cooling period was selectedfrom 7 minutes, 14 minutes, and 21 minutes, as set forth in TABLE 9below. The pucks were then electron beam irradiated according to MethodA, set forth in TABLE 2 above, at a selected dose rate until a selectedtotal irradiation dose was administered. The dose rate was selected from30 kGy-m/min, 52.5 kGy-m/min, and 75 kGy-m/min and the total irradiationdose was selected from 160 kGy, 175 kGy, and 190 kGy.

Pin-on-disc (POD) specimens in the form cylinders having a 9 mm diameterand 13 mm thickness were then machined from the UHMWPE blend pucks. Abidirectional pin-on-disc wear tester was then used to measure the wearrate of UHMWPE pins articulating against polished cobalt-chrome discslubricated by 100% bovine serum. These measurements were made inaccordance with the teachings of Bragdon, C. R., et al., in A newpin-on-disk wear testing method for simulating wear of polyethylene oncobalt-chrome alloy in total hip arthroplasty, published in the Journalof Arthroplasty, Vol. 16, Issue 5, 2001, on pages 658-65, the entiredisclosure of which is expressly incorporated by reference herein. Thebidirectional motion for the pin-on-disc wear tester was generated by acomputer controlled XY table, available from the Compumotor Division ofParker Hannifin of Cleveland, Ohio, which was programmed to move in a 10mm by 5 mm rectangular pattern. Affixed atop the XY table was a basincontaining six cobalt-chrome discs polished to an implant qualityfinish. The XY table and basin were mounted on a servo-hydraulic MTSmachine, available from MTS of Eden Prairie, Minnesota. The MTS machinethen loaded the UHMWPE blend pin specimens against the polishedcobalt-chrome discs.

The MTS machine was programmed to produce a Paul-type curve insynchronization with the motion of the XY table. A Paul-type curve isexplained in detail in Forces Transmitted By Joints in the Human Body byJ. P. Paul and published by in the Proceedings Institution of MechanicalEngineers at Vol. 181, Part 37, pages 8-15, the entire disclosure ofwhich is expressly incorporated by reference herein. The peak load ofthe Paul-type loading curve corresponded to a peak contact pressure of6.5 MPa between each of the UHMWPE pin specimens and the cobalt-chromediscs. Tests were conducted at 2 Hz to a total of 1.128×10⁶ cycles.Analysis of the results indicated that the wear properties are affectedby both the concentration of d/1-α-tocopherol and the total irradiationdose. Specifically, the results indicated that increasing thed/1-α-tocopherol concentration increased the wear rate of the UHMWPEblends, while increasing the total irradiation dose decreased the wearrate of the UHMWPE blends. Additionally, the results indicated that bothdose rate and the cooling period had substantially no impact on the wearrate of the UHMWPE.

TABLE 9 Wear Properties of UHMWPE Mixed with d/l-α-tocopherol Dose Pre-Rate Oven to POD heat Dose (kGy- Beam Wear Run Block (° C.) (kGy) VE %m/min.) (minutes) (mg/Mc) 1 Block 1 85 190 0.11 30 7 0.96 2 Block 1 115190 0.11 30 7 1.14 3 Block 1 115 190 0.11 30 21 0.76 4 Block 1 85 1900.11 30 21 0.81 5 Block 1 115 160 0.11 30 7 1.86 6 Block 1 85 160 0.1130 7 1.37 7 Block 1 115 160 0.11 30 21 1.53 8 Block 1 85 160 0.11 30 211.57 9 Block 1 85 160 0.22 75 7 2.94 10 Block 1 115 160 0.22 75 7 2.1511 Block 1 85 160 0.22 75 21 2.41 12 Block 1 115 160 0.22 75 21 1.96 13Block 1 115 190 0.22 75 7 2.57 14 Block 1 85 190 0.22 75 7 1.87 15 Block1 115 190 0.22 75 21 1.87 16 Block 1 85 190 0.22 75 21 2.24 17 Block 1100 175 0.165 52.5 14 0.89 18 Block 1 100 175 0.165 52.5 14 1.18 19Block 1 100 175 0.165 52.5 14 1.24 20 Block 1 100 175 0.165 52.5 14 1.27

EXAMPLE 6 Temperature Variations at the UHMWPE Blend/Substrate Interface

GUR 1050 medical grade UHMWPE powder was obtained from Ticona, havingNorth American headquarters in Florence, Ky. d/1-α-tocopherol wasobtained from DSM Nutritional Products, Ltd of Geleen, Netherlands. TheGUR 1050 was mechanically blended with the d/1-α-tocopherol using a HighIntensity Mixer, available from Eirich Machines of Gurnee, Ill. The GUR1050 resin was mixed with the d/1-α-tocopherol to create a UHMWPE blendhaving 0.2 wt. % d/1-α-tocopherol.

A portion of the UHMWPE blend was then compression molded into a block.Another portion of the UHMWPE blend was compression molded into asubstrate to create a preform. The substrate was a 70 mm diameter porousmetal substrate in the form of a near-net shape acetabular shell. Theporous metal substrate was produced using Trabecular Metal™ technologygenerally available from Zimmer, Inc., of Warsaw, Ind., and described indetail above. This process was repeated to create five differentpreforms. The preforms were then individually heated to a preheattemperature in a Grieve convection oven, available from The GrieveCorporation of Round Lake, Ill. The preheat temperature was selectedfrom 100° C., 120° C., and 125° C. Once heated to the selected preheattemperature, the preforms were irradiated using Method B, set forth inTABLE 2 above, until a total irradiation dose was received. The totalirradiation dose was selected from 50 kGy, 75 kGy, and 150 kGy.Additionally, the UHMWPE block was heated to a preheat temperature of100° C. and irradiated using Method B until a total irradiation dose of150 kGy was received by the UHMWPE block.

The temperature of the preforms was measured at the UHMWPEblend/substrate interface, at a point in the UHMWPE blend adjacent tothe UHMWPE blend/substrate interface, and at a point in the center ofthe UHMWPE blend. Each of the temperature measures were taken using aType J thermocouple. Additionally, the temperature at the center of theUHMWPE blend block was also measured using a Type J thermocouple. Basedon the results, the presence of a porous substrate resulted in highertemperature readings in the UHMWPE blend. This is likely a result ofsubstrate reaching a higher maximum temperature than the UHMWPE duringirradiation.

EXAMPLE 7 Effect of Substrate Orientation on UHMWPE Blend

GUR 1050 medical grade UHMWPE powder was obtained from Ticona, havingNorth American headquarters in Florence, Ky. d/1-α-tocopherol wasobtained from DSM Nutritional Products, Ltd of Geleen, Netherlands. TheGUR 1050 was mechanically blended with the d/1-α-tocopherol using a HighIntensity Mixer, available from Eirich Machines of Gurnee, Ill. The GUR1050 resin was mixed with the d/1-α-tocopherol to create a UHMWPE blendhaving 0.5 wt. % d/1-α-tocopherol.

A portion of the UHMWPE blend was compression molded into a substrate tocreate a preform. The substrate was a 70 mm diameter porous metalsubstrate in the form of a near-net shape acetabular shell. The porousmetal substrate was produced using Trabecular Metal™ technologygenerally available from Zimmer, Inc., of Warsaw, Ind., and described indetail above. This process was repeated to create three differentpreforms. The preforms were then heated in a convection oven to apreheat temperature of 110° C. for a minimum of 12 hours. Two of thepreforms were then irradiated using Method A, as set forth in TABLE 2above, with the substrate of one of the preforms facing the irradiationsource and the substrate of the other preform facing away from theirradiation source. With the preforms in these positions, they wereexposed to a first, 100 kGy dose of irradiation. The preforms were thenallowed to sit in ambient air for 20 minutes. After the expiration of 20minutes, the preforms were exposed to a second, 100 kGy dose ofirradiation, for a total irradiation dose of 200 kGy.

The remaining preform was irradiated using Method B, as set forth inTABLE 2 above, with the substrate of the preform facing the irradiationsource. With the preform in this position, the preform was exposed to afirst, 100 kGy dose of irradiation. The preform was then placed in aconvection oven which maintained a constant temperature of 110° C. Afterthe expiration of four hours, the preform was removed from theconvection oven and exposed to a second, 100 kGy dose of irradiation,for a total irradiation dose of 200 kGy.

Each of the preforms was then cut through the center and the substrateremoved. The UHMWPE blend was then microtomed and subjected to FTIRanalysis using a Bruker FTIR Spectrometer, available from Bruker Opticsof Billerica, Mass., to determine the TVI of the UHMWPE blend. Thisanalysis was performed on the thickest part of the specimens. A sampleof the UHMWPE blend was then subjected to DSC using a TA InstrumentsQ1000, available from TA Instruments of New Castle, Del., to determinethe percent crystallinity of the UHMWPE blend. This analysis wasrepeated for samples of the UHMWPE blend taken from different locations.

In both of the monoblocks that were irradiated with the substrate facingthe irradiation source, a band of discoloration, i.e., translucence, canbe seen along the edge of the UHMWPE blend that interfaced with thesubstrate. As shown in TABLE 11 below, the FTIR analysis showed asubstantial decline in the TVI of the UHMWPE blend at a point just pastthe interface between the UHMWPE blend and the substrate. Additionally,the percent crystallinity at a point in the center of the UHMWPE blendwas approximately 59%. The percent crystallinity decreased as the UHMWPEblend approached the interface with the substrate, with the percentcrystallinity reaching 48% in the translucent region near the UHMWPEblend/substrate interface, as shown in TABLE 12 below. In the preformthat was irradiated with the substrate facing away from the irradiationsource, the TVI of the UHMWPE blend was substantially more uniformthroughout the UHMWPE blend and the percent crystallinity varied by only2.2%. This may be a result of more uniform crosslinking occurring in thepreform in which the substrate faced away from the irradiation sourceduring irradiation.

TABLE 11 Comparison of TVI in UHMWPE Blend

TABLE 12 Percent Crystallinity of UHMWPE Blend % Crystallinity at the %Crystallinity at the center of the UHMWPE Blend/ Specimen UHMWPE BlendSubstrate Interface Substrate Toward 59.29% 48.67% Irradiation SourceSubstrate Toward 58.60% 47.96% Irradiation Source Substrate Away from59.88% 57.66% Irradiation Source

EXAMPLE 8 Effect of Irradiation Dose on UHMWPE Blend

Design Expert 6.0.10 software, obtained from Stat-Ease, Inc.Minneapolis, Minn., was utilized to setup a central composite responsesurface Design of Experiment (DOE). The DOE evaluated three differentvariables: d,1-α-tocopherol concentration, preheat temperature, totalirradiation dose administered, and irradiation dose per pass.

GUR 1050 medical grade UHMWPE powder was obtained from Ticona, havingNorth American headquarters in Florence, Ky. d/1-α-tocopherol wasobtained from DSM Nutritional Products, Ltd of Geleen, Netherlands. TheGUR 1050 was mechanically mixed with the d/1-α-tocopherol using a HighIntensity Mixer, available from Eirich Machines of Gurnee, Ill. The GUR1050 resin was mixed with the d/1-α-tocopherol in several batches tocreate UHMWPE blends having a selected wt. % of d/1-α-tocopherol. Thewt. % of d/1-α-tocopherol was selected from 0.10 wt. %, 0.20 wt. %, 0.35wt. %, 0.50 wt. %, and 0.60 wt. % d/1-α-tocopherol. Each of the blendswas then compression molded into a substrate to create a preform. Thesubstrate was a 70 mm outer diameter porous metal substrate in the formof a near-net shape acetabular shell. The porous metal substrate wasproduced using Trabecular Metal™ technology generally available fromZimmer, Inc., of Warsaw, Ind., and described in detail above.

The resulting preforms were then placed inside a piece of expandablebraided polyethylene terephthalate sleeving and vacuum sealed inside analuminum-metallized plastic film pouch, such a pouch formed from apolyethylene terephthalate resin, such as Mylar®, which has been coatedwith a metal, such as aluminum, to reduce gas diffusion rates throughthe film. Mylar is a registered trademark of DuPont Teijin Films U.S.Limited Partnership of Wilmington, Del. The preforms remained in thiscondition until they were removed in preparation for exposing thepreforms to irradiation. Prior to irradiation, each of the resultingpreforms was preheated by heating the preforms in a Grieve convectionoven, available from The Grieve Corporation of Round Lake, Ill., to apreheat temperature, which was held for a minimum of 12 hours. Thepreheat temperature was selected from 60° C., 70° C., 85° C., 100° C.,and 110° C., as set forth in TABLE 13 below.

The preforms were then exposed to a selected total irradiation doseaccording to Method B, as set forth above in TABLE 2. The totalirradiation dose was selected from 133 kGy, 150 kGy, 175 kGy, 200 kGy,and 217 kGy. Additionally, the total irradiation dose was divided andadministered to the preforms in either two equal passes or three equalpasses, which are combined to achieve the total irradiation dose.Specifically, the preforms indicated to be “Block 1 ” in TABLE 13 belowreceived the total irradiation dose in two equal passes, while thepreforms indicated to be “Block 2” in TABLE 13 received the totalirradiation dose in three equal passes.

After irradiation, each of the UHMWPE blends was separated from thesubstrate and three Pin-on-Disc (POD) specimens in the shape ofcylinders having a 9 mm diameter and 13 mm thickness were then machinedfrom the UHMWPE blend pucks. A bidirectional pin-on-disc wear tester wasthen used to measure the wear rate of UHMWPE pins articulating againstpolished cobalt-chrome discs lubricated by 100% bovine serum. Thesemeasurements were made in accordance with the teachings of Bragdon, C.R., et al., in A new pin-on-disk wear testing method for simulating wearof polyethylene on cobalt-chrome alloy in total hip arthroplasty,published in the Journal of Arthroplasty, Vol. 16, Issue 5, 2001, onpages 658-65, the entire disclosure of which is expressly incorporatedby reference herein. The bidirectional motion for the pin-on-disc weartester was generated by a computer controlled XY table, available fromthe Compumotor Division of Parker Hannifin of Cleveland, Ohio, which wasprogrammed to move in a 10 mm by 5 mm rectangular pattern. Affixed atopthe XY table was a basin containing six cobalt-chrome discs polished toan implant quality finish. The XY table and basin were mounted on aservo-hydraulic MTS machine, available from MTS of Eden Prairie,Minnesota. The MTS machine then loaded the UHMWPE blend pin specimensagainst the polished cobalt-chrome discs.

The MTS machine was programmed to produce a Paul-type curve [2] insynchronization with the motion of the XY table. A Paul-type curve isexplained in detail in Forces Transmitted By Joints in the Human Body byJ. P. Paul and published in the Proceedings Institution of MechanicalEngineers at Vol. 181, Part 37, pages 8-15, the entire disclosure ofwhich is expressly incorporated by reference herein. The peak load ofthe Paul-type loading curve corresponded to a peak contact pressure of6.5 MPa between each of the UHMWPE pin specimen and the cobalt-chromediscs. Tests were conducted at 2 Hz to a total of 1.128×10⁶ cycles.

The remaining portions of the UHMWPE blends were cut in half to formmicrotome films that were subjected to FTIR analysis utilizing a BrukerOptics FTIR Spectrometer, available from Bruker Optics of Billerica,Mass. The films were then accelerated aged according to ASTM StandardF2003, Standard Guide for Accelerated Aging of Ultra-High MolecularWeight Polyethylene. The OI of the post-aged films was then measured.

Once the measurements were taken, the post-aged films were placed inboiling hexane for 24 hours to extract any d/1-α-tocopherol remaining inthe films. The percentage of d/1-α-tocopherol extracted from the UHMWPEblend films was then determined. The remaining UHMWPE blend from themonoblock was then machined into 1/16″ flats and Type V tensilespecimens, as defined by ASTM Standard D638, Standard Test Method forTensile Properties of Plastics, were machined from the flats.

An analysis of the results, set forth below in TABLE 13, indicated thatwear increased with a lower total irradiation dose or with a higherconcentration of d/1-α-tocopherol. Additionally, the d/1-α-tocopherolconcentration had a significant impact on ultimate tensile elongation.The yield strength was affected the most by the preheat temperature,whereas UTS was affected the most by the total irradiation dose andd/1-α-tocopherol concentration. The OI was decreased with higher preheattemperatures and higher concentration of d/1-α-tocopherol. Although thepercentage of d/1-(α-tocopherol decreased after irradiation and aging, asignificant amount of d/1-α-tocopherol still remained in the UHMWPEblend after irradiation and aging.

TABLE 13 Effect of Irradiation Dose on UHMWPE Blend Preheat Dose PODWear Run Block (° C.) (kGy) VE % (mg/Mc) 1 Block 1 100.00 200.00 0.201.01 2 Block 1 100.00 150.00 0.50 3.84 3 Block 1 100.00 150.00 0.20 1.594 Block 1 100.00 200.00 0.50 1.78 5 Block 2 59.77 175.00 0.35 1.97 6Block 1 70.00 150.00 0.20 1.76 7 Block 1 70.00 200.00 0.20 0.80 8 Block1 70.00 150.00 0.50 3.91 9 Block 1 70.00 200.00 0.50 2.38 10 Block 2110.23 175.00 0.35 2.05 11 Block 2 85.00 132.96 0.35 3.32 12 Block 285.00 175.00 0.60 2.34 13 Block 2 85.00 175.00 0.10 0.58 14 Block 285.00 175.00 0.35 2.28 15 Block 2 85.00 217.04 0.35 1.06 16 Block 185.00 175.00 0.35 1.94 17 Block 2 85.00 175.00 0.35 2.30 YS UTS VE % OIRun Elongation % (MPa) (MPa) (Aged) (Aged) 1 248.90 21.86 41.18 0.040.04 2 306.40 22.85 47.62 0.27 0.02 3 268.10 22.50 46.06 0.07 0.03 4293.00 22.03 43.03 0.25 0.02 5 261.80 24.45 49.27 0.12 0.08 6 248.1023.08 45.95 0.06 0.06 7 223.00 23.14 43.93 0.05 0.07 8 310.00 24.0451.23 0.25 0.03 9 272.30 23.86 48.34 0.24 0.02 10 273.20 23.76 46.950.17 0.04 11 288.90 23.92 49.37 0.17 0.04 12 289.60 24.37 49.24 0.290.04 13 213.20 23.21 45.01 −0.01 0.06 14 258.80 23.97 47.60 0.17 0.05 15234.00 24.41 45.00 0.13 0.06 16 269.70 23.39 48.64 0.14 0.02 17 264.1023.95 48.41 0.15 0.05

EXAMPLE 9 Elution in Deionized Water

The amount of d/1-α-tocopherol eluted from UHMWPE blends formed intoconsolidated pucks was investigated over a period of 8 weeks. GUR 1050medical grade UHMWPE powder was obtained from Ticona, having NorthAmerican headquarters in Florence, Ky. d/1-α-tocopherol was obtainedfrom DSM Nutritional Products, Ltd of Geleen, Netherlands. The GUR 1050was mechanically mixed with the d/1-α-tocopherol using a High IntensityMixer, available from Eirich Machines of Gurnee, Ill. The GUR 1050 resinwas mixed with the d/1-α-tocopherol to create a UHMWPE blend having 0.25wt. % of d/1-α-tocopherol. The UHMWPE blend was then compression moldedinto a series of 2.5 inch diameter and 1.5 inch thick pucks.

The pucks were preheated in a Grieve convection oven, available from TheGrieve Corporation of Round Lake, Ill., to a preheat temperature. Thepreheat temperature was selected from 85° C. and 115° C. . Oncepreheated, the pucks were then exposed to a selected total irradiationdose according to Method A, as set forth above in TABLE 2. The totalirradiation dose was selected from 160 kGy and 190 kGy. One centimetercubes were then machined from the pucks and placed in glass jarscontaining 100 ml of deionized water. The jars were then sealed usingTeflon® seals and caps, available from E.I. DuPont Nemours and Company.Teflon® is a registered trademark of E. I. DuPont Nemours and Company of1007 Market Street, Wilmington Del.

Each of the glass jars was then placed in a water bath that wasthermostatically held at a test temperature. The test temperature wasselected from 37° C. and 70° C. At two week intervals, aliquots ofextract solution were taken from each jar and assayed using 297 nmwavelength ultraviolet light to determine the concentration ofd/1-α-tocopherol. Absorption measurements were made using 10 mm quartzcuvettes and deionized water as the reference material. Once the assaywas completed, the test aliquots were returned to the glass jars. Thisanalysis was repeated for a total of 53 days. As the results set forthbelow in TABLE 14 indicate, no eluted d/1-α-tocopherol was detected inthe UHMWPE blend cubes that were soaked in deionized water maintained at37° C. Additionally, no definitive elution of d/1-α-tocopherol wasdetected in the UHMWPE blend cubes that were soaked in deionized watermaintained at 70° C. For example, the results showed that theantioxidant leached from 2 grams of the crosslinked UHMWPE in 100milliliters of 37 degree Celsius water after 53 days resulted in anextraction solution absorbance at 297 nanometers was no greater than0.01 units from the reference water absorbance.

TABLE 14 Elution of d/l-α-tocopherol in Deionized Water Solvent Sample53 Day Water 53 Day Water Group # Solvent Temperature (C.) Wt (g) - Vol(mL) Weight (g) Raw A @ 297 nm Net A @ 297 nm A Water 37 100 1.91335−0.0017 0.000 A Water 70 100 1.90159 0.0032 0.005 B Water 37 100 1.91722−0.0012 0.000 B Water 70 100 1.91635 0.0043 0.006 C Water 37 100 1.90948−0.0014 0.000 C Water 70 100 1.91114 0.0030 0.005 D Water 37 100 1.90083−0.0016 0.000 D Water 70 100 1.92051 0.0036 0.005 Water Blank A −0.0016

EXAMPLE 10 Color Measurement of UHMWPE Blend Samples

GUR 1050 medical grade UHMWPE powder was obtained from Ticona, havingNorth American headquarters in Florence, Ky. d/1-α-tocopherol wasobtained from DSM Nutritional Products, Ltd of Geleen, Netherlands. TheGUR 1050 was mechanically blended with the d/1-α-tocopherol using a HighIntensity Mixer, available from Eirich Machines of Gurnee, Ill. The GUR1050 resin was mixed with the d/1-α-tocopherol to create a UHMWPE blendhaving less than 0.5 wt. % d/1-α-tocopherol and was compression molded.The compression molded UHMWPE blend was then sectioned and subjected toanalysis with a spectrophotometer to determine the color of the UHMWPEblend. Additionally, consolidated UHMWPE powder absent tocopherol wasalso subjected to analysis with a spectrophotometer to determine thecolor of the consolidated UHMWPE absent tocopherol.

Specifically, a Color Checker 545 Portable Spectrophotometer hand heldunit, available from X-Rite Incorporated of Grand Rapids, Mich., wasused to test the material samples. This device uses a system illuminantD65 and has a degree observer, i.e., the placement of the devicerelative to the sample being tested, of 10 degrees. The device wascalibrated using a calibration tile and the average results per readingwere recorded for comparison with the test samples. Each of the sampleswere then subjected to analysis.

The results of each individual analysis were displayed on the deviceusing the L*a*b* (CIELAB) color space definition system. This systemdescribes all colors visible to the human eye by providing the lightnessof the color, the position of the color between red/magenta and green,and the position of the color between yellow and blue. These results aredisplayed as L*, having a value from 0, which corresponds to black, to100, which corresponds to white, a*, where a negative value indicatesgreen and a positive value indicates red/magenta, and b*, where anegative value indicates blue and a positive value indicates yellow.

Based on the results of the testing, set forth in TABLE 15 below, theUHMWPE blend having tocopherol exhibited a yellowish color.

TABLE 15 Color Measurements of UHMWPE and UHMWPE w/ Tocopherol MaterialL* a* b* UHMWPE 96.58 −8.30 19.53 UHMWPE 96.72 −8.34 19.49 UHMWPE 96.01−9.48 18.28 UHMWPE Blend 96.15 −7.93 20.48 (0.5 VE %) UHMWPE Blend 96.79−8.00 20.56 (0.5 VE %) UHMWPE Blend 95.40 −9.10 19.23 (0.5 VE %)

EXAMPLE 11 Swell Ratio, Crosslink Density, and Molecular Weight BetweenCrosslinks

GUR 1050 medical grade UHMWPE powder was obtained from Ticona, havingNorth American headquarters in Florence, Ky. d/1-α-tocopherol wasobtained from DSM Nutritional Products, Ltd of Geleen, Netherlands. TheGUR 1050 was mechanically blended with the d/1-α-tocopherol using a HighIntensity Mixer, available from Eirich Machines of Gurnee, Ill. The GUR1050 resin was mixed with the d/1-α-tocopherol to create UHMWPE blendshaving 0.2, 0.5, or 1.0 weight percent d/1-α-tocopherol. The UHMWPEblends were then compression molded to form pucks that were thenmachined to form cubes having 5 mm sides. The UHMWPE cubes were thenheated to a preheat temperature selected from 40° C., 100° C., and 110°C. Once heated to the selected preheat temperature, the UHMWPE blendswere irradiated using Method C, set forth in TABLE 2 above, until atotal irradiation dose was received. The total irradiation dose wasselected from of 90 kGy, 120 kGy, 150 kGy, and 200 kGy.

The resulting UHMWPE blend cubes were then studied to investigate thepolymer network parameters of the UHMWPE blend by measuring thematerials' swell ratio (q_(S)) with a Swell Ratio Tester (SRT),Cambridge Polymer Group (Boston, Mass.), in accordance with ASTMF-2214-02. Knowing q_(S), the Flory interaction parameter (χ₁), themolar volume of the solvent (φ₁), and the specific volume of the solvent( v), the crosslink density (u_(X)) and the molecular weight betweencrosslinks (M_(C)) of the material were calculated according thefollowing equations:

$v_{x} = {- \frac{{\ln \left( {1 - q_{s}^{- 1}} \right)} + q_{s}^{- 1} + {\chi_{1}q_{s}^{- 2}}}{\phi_{1}\left( {q_{s}^{{- 1}/3} - {q_{s}^{- 1}/2}} \right)}}$$M_{c} = {\overset{\_}{v}v_{x}}$

Additionally, the swell ratio in stabilized o-xylene at 130° C. wasmeasured in the compression molded direction. The results of the testingare set forth in TABLE 16 below. For example, it was found that a UHMWPEblend having nominally 1.0% weight percent of d/1-α-tocopherol whenpreheated to nominally 40° C. and subsequently electron beam crosslinkedwith a total dose of nominally 200 kGy has a q_(S) less than about 4.3,a u_(X) more than about 0.090 and a M_(C) less than about 11,142. It wasalso found that a UHMWPE blend having nominally 1.0% weight percent ofd/1-α-tocopherol when preheated to nominally 110° C. and subsequentlyelectron beam crosslinked with a total dose of nominally 200 kGy has aq_(S) less than about 3.6, a u_(X) more than about 0.117 and a M_(c)less than about 8,577.

Also, it was found that a UHMWPE blend having nominally 0.5% weightpercent of d/1-α-tocopherol when preheated to nominally 40° C. andsubsequently electron beam crosslinked with a total dose of nominally200 kGy has a q_(S) less than about 3.8, a u_(x) more than about 0.119and a M_(C) less than about 8,421. It was also found that a UHMWPE blendhaving nominally 0.5% weight percent of d/1-α-tocopherol when preheatedto nominally 110° C. and subsequently electron beam crosslinked with atotal dose of nominally 200 kGy has a q_(S) less than about 3.6, a u_(X)more than about 0.109 and a M_(c) less than about 9,166.

Further, it was found that a UHMWPE blend having nominally 0.2% weightpercent of d/1-α-tocopherol when preheated to nominally 40° C. andsubsequently electron beam crosslinked with a total dose of nominally200 kGy has a q_(S) less than about 2.8, a u_(X) more than about 0.187and a M_(C) less than about 5,351. It was also found that the UHMWPEblend having nominally 0.2% weight percent of d/1-α-tocopherol whenpreheated to nominally 110° C. and subsequently electron beamcrosslinked with a total dose of nominally 200 kGy has a q_(S) less thanabout 3.0, a u_(X) more than about 0.164 and a M_(c) less than about6,097.

Additionally, it was found that under some conditions the crosslinkedUHMWPE blend exhibited a crosslink density of less than 0.200 moles/dm³.Under other conditions, the crosslinked UHMWPE blend having at least 0.1weight percent antioxidant exhibited a crosslink density of less than0.190 moles/dm³. Further, under certain conditions, the crosslinkedUHMWPE blend having at least 0.1 weight percent antioxidant exhibited acrosslink density of more than 0.200 moles/dm³ and had a molecularweight between crosslinks of less than 11,200 daltons.

TABLE 16 Swell Ratio, Crosslink Density, and Molecular Weight BetweenCrosslinks PRE- PER- DOSE SAMPLE HEAT CENT RATE MATERIAL DOSE TEMPVITAMIN (kGy-m/ RUN TYPE (kGy) (° C.) E min) 1 GUR 1020 90 40 0.2 75.002 GUR 1050 90 100 0.2 75.00 3 GUR 1050 150 40 0.2 75.00 4 GUR 1020 150100 0.2 75.00 5 GUR 1020 90 40 1.0 75.00 6 GUR 1020 90 100 0.5 75.00 7GUR 1020 150 40 0.5 75.00 8 GUR 1020 150 100 1.0 75.00 9 GUR 1050 90 400.2 240.00 10 GUR 1020 90 100 0.2 240.00 11 GUR 1020 150 40 0.2 240.0012 GUR 1050 150 100 0.2 240.00 13 GUR 1050 90 40 1.0 240.00 14 GUR 102090 100 1.0 240.00 15 GUR 1020 150 40 1.0 240.00 16 GUR 1020 150 100 0.5240.00 17 GUR 1050 120 40 0.5 157.50 18 GUR 1050 120 100 1.0 157.50 19GUR 1050 120 40 1.0 157.50 20 GUR 1050 120 100 1.0 157.50 21 GUR 1050 9040 1.0 75.00 22 GUR 1050 90 100 0.5 75.00 23 GUR 1050 150 40 0.5 75.0024 GUR 1050 150 100 1.0 75.00 25 GUR 1050 90 40 0.5 240.00 26 GUR 105090 100 1.0 240.00 27 GUR 1050 150 40 1.0 240.00 28 GUR 1050 150 100 0.5240.00 29 GUR 1050 2 × 100 = 200 40 0.2 240.00 30 GUR 1050 2 × 100 = 200110 0.2 240.00 31 GUR 1050 2 × 100 = 200 40 1.0 240.00 32 GUR 1050 2 ×100 = 200 110 1.0 240.00 33 GUR 1050 2 × 100 = 200 40 0.5 240.00 34 GUR1050 2 × 100 = 200 110 0.5 240.00 POD WEAR SWELL mg/1M RATIO Vx = XLD Mc= MWbXL RUN CYCLES V/V0 = q(s) X moles/dm{circumflex over ( )}3 Daltons1 5.09 0.44 0.068 14747 2 3.40 0.49 0.129 7764 3 2.65 3.15 0.50 0.1476812 4 1.13 4.61 0.45 0.079 12652 5 6.15 0.42 0.051 19720 6 5.52 0.430.060 16706 7 4.22 0.46 0.091 11019 8 5.52 0.43 0.060 16706 9 3.75 0.480.110 9126 10 4.49 0.45 0.082 12143 11 3.84 0.47 0.105 9483 12 0.17 3.230.50 0.141 7115 13 7.13 0.41 0.040 24747 14 4.47 0.45 0.083 12059 155.69 0.43 0.057 17502 16 4.46 0.45 0.083 12017 17 4.50 0.45 0.082 1218518 3.74 0.48 0.110 9087 19 5.32 0.43 0.063 15785 20 3.33 0.50 0.133 749621 5.78 0.43 0.056 17929 22 4.43 0.45 0.084 11891 23 3.92 3.84 0.470.105 9483 24 3.88 0.47 0.104 9642 25 5.59 0.43 0.059 17033 26 4.52 0.450.082 12270 27 4.37 0.46 0.086 11640 28 1.63 3.65 0.48 0.115 8733 290.02 2.76 0.53 0.187 5351 30 0.09 2.96 0.52 0.164 6097 31 1.46 4.25 0.460.090 11142 32 0.64 3.61 0.48 0.117 8577 33 3.57 0.48 0.119 8421 34 3.760.48 0.109 9166

1. A method for processing UHMWPE for use in medical applications, themethod comprising the steps of: combining UHMWPE with an antioxidant toform a blend having 0.01 to 3.0 weight percent of the antioxidant;processing the blend to consolidate, the consolidated blend having amelting point; preheating the consolidated blend to a preheattemperature below the melting point of the consolidated blend; andirradiating the consolidated blend while maintaining the consolidatedblend at a temperature below the melting point of the consolidatedblend.
 2. The method of claim 1, wherein said combining step furthercomprises combining UHMWPE with an antioxidant to form a substantiallyhomogenous UHMWPE blend having 0.01 to 3.0 weight percent of theantioxidant.
 3. The method of claim 1, wherein said combining stepfurther comprises mixing the UHMWPE powder with tocopherol by at leastone of solvent blending, high shear mixing, precision coating, fluidizedbed, atomization, emulsion polymerization, electrostatic precipitation,wetting or coating of particles, and master batch blending.
 4. Themethod of claim 1, wherein said combining step further comprises mixingthe UHMWPE powder with tocopherol by high shear mixing.
 5. The method ofclaim 1, wherein said combining step further comprises combining theUHMWPE with an antioxidant selected from the group consisting oftocopherol, Vitamin C, lycopene, honey, phenolic antioxidants, amineantioxidants, hydroquinone, beta-carotene, ascorbic acid, CoQ-enzyme,and derivatives thereof.
 6. The method of claim 1, wherein saidcombining step further comprises combining the UHMWPE withd,1-α-tocopherol.
 7. The method of claim 1, wherein said processing stepfurther comprises processing the blend to consolidate the blend by atleast one of compression molding, net shape molding, injection molding,extrusion, monoblock formation, fiber, melt spinning, blow molding,solution spinning, hot isostatic pressing, high pressurecrystallization, and films.
 8. The method of claim 1, wherein saidprocessing step further comprises processing the blend to consolidate bycompression molding the blend to extend at least partially into a poroussubstrate.
 9. The method of claim 8, wherein the substrate comprises ahighly porous biomaterial useful as at least one of a bone substitute, acell receptive material, a tissue receptive material, an osteoconductivematerial, and an osteoinductive material.
 10. The method of claim 8,wherein the substrate comprises a highly porous biomaterial useful as atleast one of a bone substitute, a cell receptive material, and a tissuereceptive material having a porosity of at least 55 percent.
 11. Themethod of claim 1, wherein said processing step further comprisesprocessing the blend to consolidate by compression molding the blendinto a roughened surface on the substrate having macroscopicmechanically interlocking features.
 12. The method of claim 1, whereinthe melting point of the consolidated blend is determined bydifferential scanning calorimetry.
 13. The method of claim 1, whereinsaid preheating step further comprises preheating the consolidated blendto a preheat temperature substantially between 70 degrees Celsius and130 degrees Celsius.
 14. The method of claim 1, further comprising theadditional step of machining the consolidated blend to form a medicalproduct.
 15. The method of claim 1, further comprising the additionalsteps of: identifying a total crosslinking irradiation dose to bereceived by the consolidated blend; identifying a maximum individualirradiation dose to be received by the consolidated blend whilemaintaining the consolidated blend below the melting point of theconsolidated blend; and wherein the step of irradiating the consolidatedblend further comprises irradiating the consolidated blend with a firstirradiation dose that is one of equal to and less than the maximumindividual irradiation dose.
 16. The method of claim 15, furthercomprising the additional step of irradiating the consolidated blendwith at least one subsequent irradiation dose that is one of equal toand less than the maximum individual irradiation dose.
 17. The method ofclaim 15, wherein said step of irradiating the consolidated blendfurther comprises irradiating the consolidated blend with at least oneof electron beam irradiation, gamma irradiation, and x-ray irradiation.18. The method of claim 15, wherein the total crosslinking irradiationdose is substantially between 50 kGy and 1000 kGy.
 19. The method ofclaim 15, wherein the total crosslinking irradiation dose issubstantially between 100 kGy and 250 kGy.
 20. The method of claim 15,further comprising, after said first irradiating step, the additionalsteps of: equilibrating the temperature of the consolidated blend to thepreheat temperature; and irradiating the consolidated blend with asecond irradiation dose that is one of equal to and less than the lesserof the difference between the total crosslinking irradiation dose andthe first irradiation dose and the maximum individual irradiation dosefor the consolidated blend.
 21. The method of claim 1, furthercomprising the additional step of sterilizing the UHMWPE blend by atleast one of gas plasma sterilization, ethylene oxide sterilization,gamma sterilization, ionizing irradiation sterilization, autocalving,and supercritical fluid techniques.
 22. The method of claim 1, furthercomprising, after said irradiating step, the additional step of heattreating the consolidated blend.
 23. The method of claim 1, furthercomprising, before said irradiating step, the additional step ofshielding at least a portion of the consolidated blend.
 24. Acrosslinked UHMWPE blend for use in medical implants prepared by aprocess comprising the steps of: combining UHMWPE with an antioxidant toform a blend having 0.1 to 3.0 weight percent antioxidant; processingthe blend to consolidate the blend, the consolidated blend having amelting point; preheating the consolidated blend to a preheattemperature below the melting point of the consolidated blend; andirradiating the consolidated blend with a total irradiation dose of atleast 100 kGy while maintaining the consolidated blend at a temperaturebelow the melting point of the consolidated blend.
 25. The crosslinkedUHMWPE blend of claim 24, wherein the crosslinked UHMWPE blend has anultimate tensile elongation of at least 250 percent.
 26. The crosslinkedUHMWPE blend of claim 24, wherein the crosslinked UHMWPE blend has atensile yield strength of at least 21 mega pascals.
 27. The crosslinkedUHMWPE blend of claim 24, wherein the crosslinked UHMWPE blend has anultimate tensile strength of at least 45 mega pascals.
 28. Thecrosslinked UHMWPE blend of claim 24, wherein the crosslinked UHMWPEblend has a pin-on-disc wear rate of less than 2.75 mg/Mc.
 29. Thecrosslinked UHMWPE blend of claim 24, wherein the crosslinked UHMWPEblend has an izod impact strength of at least 53 kJ/m .
 30. Thecrosslinked UHMWPE blend of claim 24, wherein the crosslinked UHMWPEblend is molded into at least one of a substrate, a polymer, and aantioxidant stabilized polymer.
 31. The crosslinked UHMWPE blend ofclaim 24, wherein the crosslinked UHMWPE blend has an oxidation index,the oxidation index remaining below 0.1 after accelerated aging for twoweeks in an oxygen atmosphere at a pressure of 72 pounds per square inchand a temperature of 70 degrees Celsius.
 32. The crosslinked UHMWPEblend of claim 24, wherein the antioxidant leached from 2 grams of thecrosslinked UHMWPE in 100 milliliters of 37 degree Celsius water after53 days results in an extraction solution absorbance at 297 nanometersthat is no greater than 0.01 units from the reference water absorbance.33. The crosslinked UHMWPE blend of claim 24, wherein the antioxidantleached from 2 grams of the crosslinked UHMWPE in 100 milliliters of 70degree Celsius water after 53 days results in an extraction solutionabsorbance at 297 nanometers that is no greater than 0.01 units from thereference water absorbance.
 34. The crosslinked UHMWPE blend of claim24, wherein the crosslinked UHMWPE blend has an oxidation index, theoxidation index being less than 0.1 percent and the crosslinked UHMWPEblend has a free radical level measured by electron spin resonance thatis less than 1×10¹⁷ spins per gram.
 35. The crosslinked UHMWPE blend ofclaim 24, wherein the crosslinked UHMWPE blend has an oxidation index,the oxidation index being less than 0.1 percent, and wherein thecrosslinked UHMWPE exhibits a crosslink density of more than 0.200moles/dm³.
 36. The crosslinked UHMWPE blend of claim 24, wherein thecrosslinked UHMWPE blend has at least 0.1 weight percent antioxidant andexhibits a pin-on-disc wear rate of less than 2.0 mg/Mc.
 37. Thecrosslinked UHMWPE blend of claim 24, wherein the crosslinked UHMWPEexhibits a crosslink density of more than 0.200 moles/dm³.
 38. Thecrosslinked UHMWPE blend of claim 24, wherein the crosslinked UHMWPEblend has at least 0.1 weight percent antioxidant and exhibits acrosslink density of more than 0.190 moles/dm³.
 39. The crosslinkedUHMWPE blend of claim 24, wherein the crosslinked UHMWPE blend has atleast 0.1 weight percent antioxidant, exhibits a crosslink density ofmore than 0.200 moles/dm³, and has a molecular weight between crosslinksof less than 11,200 daltons.
 40. The crosslinked UHMWPE blend of claim24, wherein the UHMWPE blend has an L* value, an a* value, and a b*value as measured by a colormeter, wherein the L* value is greater than90, the a* value is greater than negative 6, and the b* value is greaterthan
 17. 41. The crosslinked UHMWPE blend of claim 24, wherein theprocess further comprises the additional step of molding theconsolidated blend into at least one of a porous substrate, a polymer,and a polymer containing an antioxidant.
 42. The crosslinked UHMWPEblend of claim 24, wherein the crosslinked UHMWPE blend has a freeradical level measured by electron spin resonance that is less than1×10¹⁷ spins per gram.